At Thanksgiving, what was your DHs response to him?
Honestly, I find myself thinking a similar thing about gifting as what you're saying Jeff is saying - I'm more than happy to cut gifts out at Christmas. And I would be taken aback if someone offered to put my name on a childs birthday gift - that is expressing that this is a gifting obligation in some form, especially coming from your father. Age doesn't matter much in that respect, imo.
The conversation I think your husband should have with him shouldn't be about reminders, but a talk about the gifts - what does Jeff want to do? And if he does not want Christmas gifts and to stay out of that exchange, I think he is looking for his father to agree to this so there is not the obligation to gift him back. It sounds like Jeff is looking for this agreement and in not getting one, he feels obligated to gift when he just doesn't want to.
At Thanksgiving, DH responded affirmatively to Jeff about Christmas. We were ready to respect what Jeff said he wanted to do. Given that, we were surprised when Jeff asked for Christmas gift ideas. For us, at that point, it came from left field. We hadn't planned to buy anything. Like I said, we scrambled to buy a few items so we could reciprocate and get them in the mail. I don't know what Jeff's mom said over Thanksgiving about Christmas, but I do know that Jeff blew up about it. (We weren't there for that blow-up. This was relayed to DH by Jeff's stepdad.)
I said in the OP that DH and Jeff spoke for an hour a couple of weeks ago about the issue of gifting. The discussion focused on Jeff's feelings about it, which DH said were very confusing, and that may be why it isn't coming across well here. Among the things Jeff said was that he resented "having to buy gifts for Christmas." As far as we were concerned, at Thanksgiving, he made his statement about Christmas gifting, and we intended to respect that. No one made him buy gifts at Christmas. He asked for ideas, which confused us.
It may help to think about this on a timeline:
Thanksgiving: Jeff makes his statement about Christmas gifting, which was, "I'm in my mid-20s now, and I feel like I'm too old for giving and receiving Christmas gifts." DH understands Jeff's position.
Ten days before Christmas: Jeff asks for gift ideas. We are confused, but we provide gift ideas. We then run out to buy gifts for Jeff and his GF so we can mail them before Christmas.
A few days before Christmas: Gifts from Jeff arrive at our house.
Christmas Day: We open gifts from Jeff. DH receives both gift ideas I'd suggested; DS gets a $50 gift card; I received the gloves. We talked by phone and thanked each other for the gifts.
Two weeks after Christmas: DH runs into Jeff's stepdad. Stepdad asks DH if we had any interaction with Jeff at Thanksgiving about Christmas gifts. DH tells Stepdad about Jeff's comment that he is too old to give and receive gifts. Stepdad tells DH that Jeff became angry at Thanksgiving when they were discussing plans for Christmas. Stepdad's concern was about how angry Jeff was about Christmas.
Three weeks after Christmas: One of my stepsons asks DH if DS's birthday is coming up. DH texts all three of them about DS's upcoming birthdate and asks if they would be interested in putting their names on a gift we already bought. Two respond positively. Jeff is angry. He tells DH by text that every time some event comes up, he feels that he's expected to give a gift. All of this -- Jeff's reaction and his position about gifting in general -- was news to DH. DH had no idea Jeff felt that way. DH agrees to call Jeff to talk about it.
That night: DH calls Jeff. They talk for an hour. Jeff complains that he doesn't like having to give all these gifts. Every time someone has an occasion, it's as if he is expected to give a gift. He doesn't think he should have to participate in putting his name on a gift to DS because as an adult he doesn't gift his adult brothers. He complains that he "had to give gifts at Christmas" to the person he asked for gift ideas. It was a lot of "I don't think I should have to." DH confirmed for Jeff that in fact he doesn't have to. Gifting is his decision. People give gifts because they want to. If he doesn't want to, then he shouldn't. Jeff said he understood that. Then the discussion shifted to how Jeff doesn't like being told about these life events because it makes him feel as if a gift is expected. Does Jeff not want information about life events? No, he does want information about life events. He loves us and wants relationships
with us. He doesn't want to overlook events. It was all very confusing. When they got off the phone, DH came into the room where I was and was just shaking his head. Until that phone call we had no idea that Jeff felt such resentment about an expectation that gifts might be given for life events. We also didn't know that he gets heartburn over being told that these life events are taking place.
So, while Jeff's feelings about being asked to participate in DS's birthday gift are understandable now after the phone call, there was no way DH could have predicted that when he asked Jeff about putting his name on the gift. Jeff had never articulated his feelings about gifting in general until that phone call. Until that call, our only interaction with Jeff about gifts was the comment he made at Thanksgiving about being too old for Christmas gifts.
Regardless, from our perspective, we are having difficulty with the idea that he resents giving gifts. He absolutely should not give gifts if doing that stirs up resentment for him. This is why I've been saying that he needs to live his values where gifting is concerned. He has to decide what he wants to do about gifting, and we will respect that. But he has to decide. Whatever he wants to do, we will respect. But it's not okay to give gifts and then complain about it afterward to the people you gifted.