It sounds like the "special occasion" bomb has gone off amongst his peer group. It seemed like that once I hit 20/21 your friends are graduating, birthdays, weddings, babies, and then add in keeping up with immediate and extended family!
It might be time for an in-person conversation to find out what's going on. If it was me, I would do this in person, not over the phone or via email. He might need some help figuring out how to navigate all of the social obligations that are probably coming his way. He needs to realize that though you might be invited to something, you don't necessarily need to attend nor send a gift (but some appropriate recognition (card/letter) of the occasion may still be needed. He might also not feel comfortable figuring out what kind of gifts to give and what are appropriate to each kind of occasion (coworkers wedding vs. best friends wedding). You might even be able to tie a conversation into how you and your husband as a couple are re-evaluating gift giving within the larger family.
Just musing aloud here, it might also be time to figure out what events you want to mutually celebrate....do you expect him to remember your wedding anniversary, Grandma and Grandpa's?, are those people going to reciprocally recognize his milestone? Do Grandma and Grandpa not really care if he recognizes their anniversary, but you think it would be nice if he sent them a card on their 50th wedding anniversary. As a 20 something, that's what I had a hard time figuring out. Also, as I'm entering my 30's and married with child, things are changing again. Peer group birthdays and graduations are pretty much over with, or very low key "lets gather for drinks" type affairs. Now it's baby showers, weddings, and kid birthday parties, never mind navigating the land mines of extended family gift giving at Christmas and birthdays.