TootsNYC is right: DH is not at all sulky or upset around Rob and his brothers. He is genuinely happy when the boys are around. And by happy, I mean thrilled and enthusiastic. He loves to see them, even if it's for a short time. Later, he may tell me how he's feeling deep down, but it's not on display when the boys are around. And when situations like this recent one with Rob arise, we spend more time talking about how he feels.
DH and Rob are very different people with very different personalities. Their interests are pretty divergent. Despite that, I think there's something to the "wooing" idea. One thing they share in common is their love for the pro baseball team in BigCity. They talk frequently about baseball. It would be fairly easy for them to go to some games together. It would be a starting place, but I would like to see DH expand their relationship beyond baseball-talk though.
Rob is into golf. DH has never played golf. It might be possible for two of them to play golf sometime, so Rob could teach DH how to play. If they did that, I would like to see Rob and DH go without the other two boys. If the other two boys are involved, it would create a competitive atmosphere in the golf game, which adds some stress to the mix.
I'm pondering some possibilities here ...