Does your DH ever specifically invite Rob and Michelle over? Does he ever try to make specific plans with them, for them to visit him from their home in BigCity or for him to go there to visit them?
I ask because the impression that I get from the OP is that your DH waits to hear that they will happen to be in your town, and then waits around hoping that they will want to see him. Which may be giving Rob (and his other sons) that their visits are not actually important to your DH... because if they were then your DH would be doing something other than just sitting by the phone... (Actions speak louder than words, afterall...)
I think that your DH needs to be proactive and actually take the initiative to contact Rob and make plans to get together, if spending time with Rob is something that is really important to your DH. Then, if Rob continues to blow off your DH... well, there you go... but at least you will know that you made every effort....
Yes, we do invite Rob and Michelle here. It's pretty much the same pattern: We cannot get a commitment from Rob about whether they will be here. A good example happened last Thanksgiving and Christmas. At Thanksgiving, we made plans to host dinner here. DH's other two sons planned to come, all was well there. DH told all three boys what time we were serving dinner. Even as late as the day before Thanksgiving, we had no idea what Rob and Michelle were going to do. Then Rob said they would be tied up at Michelle's parents' house most of Thanksgiving day but would come by later in the afternoon. When they arrived, it was obvious that Michelle was surprised we had already served dinner. She apologized to me about missing dinner. Rob didn't say anything. I have no idea whether he was expecting us to hold dinner for them.
A few weeks before Christmas, we had tickets to see a show in BigCity for our anniversary. DH let Rob know several weeks ahead that we would be in town and hoped to get together with them for dinner that evening. Rob expressed interest. And we spent those next few weeks trying to pin Rob down. When we arrived in BigCity the day of the show, we had no idea whether we were actually going to see Rob and Michelle that evening for dinner. It wasn't until about 4 p.m. that we had confirmation from them with a meeting time for dinner and a restaurant.
For a weekend last summer, DH contacted Rob about getting together. Rob replied and told DH they were busy. Then, of all things, DH happened to run into Rob and Michelle in a restaurant in OurCity that Friday evening. Michelle was clearly mortified. Rob was apologizing all over himself. Rob texted DH later and told him that they had no idea they would run into DH at that restaurant and Michelle was very upset about the situation. I'm not sure what they expected DH to say about it.
DH's dad (Rob's grandfather) is buried in a cemetery in BigCity. Every year, we visit the cemetery for Veterans' Day. DH has asked Rob if he'd come along, and Rob always declines.
It's just how it is with Rob.
There are quite a few parallels between this thread and spookycatlady's "I didn't want to go to your stupid party anyway" thread. DH is so desperate for Rob's attention/affection that like spookycatlady's husband, he will grab on to any piddly crumbs Rob is willing to throw his way. And when he accepts those crumbs, he is teaching Rob that he doesn't have to make any effort, because his dad is satisfied with being an afterthought. Heck, given the way his dad waits by the phone, he seems to LOVE being an afterthought.
You both need to set hard boundaries about which times you're available for his flyby visits. And then you need to show some tough love with DH in terms of not letting him sit by the phone and fret. Let him know that his checking the phone, being reluctant to leave on time for commitments in case Rob calls, etc., is hurtful to you as it give you the impression that Rob's flaky attentions are more important than spending with you. Don't you two have a younger (pre-teen) son living at home? How does DH think his fretting behavior on Father's Day affected your youngest? Take away his phone if you have to and check the messages yourself. DH is letting Rob steal his dignity and reason by letting Rob treat him this way.
As for the Thanksgiving incident, I think you handled it just right. Continue to hold commitments as planned, even if it means celebrating without Rob and Michelle. She was shocked that dinner was already served? Well boohoo. They don't get to have it both ways. They can't refuse to commit to a meal time and expect to share a meal with you.
And the restaurant coincidence? I sincerely hope DH didn't apologize for Michelle's feelings being hurt. They were embarrassed because they got caught "having better things to do" to visit with you. They were mortified by you having the "gall" of making them feel bad because they couldn't make time to visit you? Boohoo again. If you behave like an inconsiderate person, be prepared to feel like a inconsiderate person.
Stay strong, Coley. I know this is hard. We love you.