Sam:
You cannot force this relationship with parents to work. I found this out the hard way. My husband's family is VERY different from mine. We are in constant contact. No kidding - I can talk to my sister several times in one day and I usually talk to my parents DAILY.
Bob's family: call when there is a problem. Otherwise - no news is good news.
To be honest - I think it sucks. So I have talked to his sister...we had a rough road...but now it's all good. She too thought it was funky that their family was "no news is good news" - she always wondered what it would be like to have someone to share things with (other than friends)...well.. she has me!!!
Bob has ONLY started talking to his dad on Sunday's this year. After going to visit his sister a few years ago - they both realized how dysfunctional their family was. Bob's mom died on Mother's Day 2006. That changed a LOT of things...so really...my point is - you can't force this. if there is a problem - he can make excuses about job - he might have unresolved feelings - I DO NOT KNOW - however - he may place family in a different priority than you. To force him to go will make for a bad trip. You can keep in contact with his family, but don't force it.
That they (his parents) have kids in sports, etc. well - sorry - it's just as easy for them to say - Johnny will miss his game this weekend. We have family in town. Because if you go there? They will still go to the game, right? So how much visiting are you going to get done? I think it's great that it sounds like they are trying to come to you...don't let anyone make assumptions...two people traveling can be just as complicated as a family of 4 traveling...time off work, expectations, etc.
I would just encourage a family relationship. Don't push it. When he's ready to go, he'll make it happen. They too are adults and can make decisions to come to see you.