One random thought--I think if parents want to be someone that their grown children will seek out to spend time with, they need to be "enjoyable people to spend time with."
Not necessarily great buddies, etc. But *all* of us, no matter what our relationship, spend time with people we want to spend time with.
So DH may have to "woo" Rob. Seek him out for things he's interested in. (Rob likes one particular music style? DH should invite him to go to a concert in that genre. / Rob plays golf? DH should suggest they go golfing.)
And then when they're together, basically DH will need to do the old "teenage girl trying to get a guy to be interested in her" conversational trick. Ask questions to get him talking about himself, be very interested in him and his life, etc. And be enjoyable to be around. On the second visit, remember and mention stuff Rob talked about in the first visit. Etc.