I agree with Cheryl O. on this one - it may just be his personality and temperment and nothing you say or do will change that. Sometimes my younger stepson could be the same way, but mostly when it came to going along with something everyone else had agreed on - we all wanted to go out for pizza, he wanted Chinese; we all wanted to see a particular movie, he wanted to see a different movie, etc. We just kept it simple, didn't let him dictate everyone else's choices, and let him know this is what the plan is, he can take it or leave it. We just assumed that no matter what was picked, he would always want something else. Once in a while we would give in just so he would not feel like he never had a say or got his way, but for the most part it was "like it or lump it".
I wouldn't stress anymore about it. I would just assume that this is what his response is going to be, and let him know it's his decision and his choice and if he opts to miss out, oh well. It's not like he has to wear a bandana for Pirate Day or he has to ride a dirt bike. It might help to remind him of all the times he's initially not wanted to do something and then ended up having a lot of fun, but in the end, it's really his issue - don't make it yours.