Laurie A., it sounds like you're dealing with a terribly unhealthy situation, but with all due respect, this doesn't look like the same thing. This is his father's 50th birthday celebration with family coming in town from several states. Wanting their son present for this special celebration doesn't mean his parents expect him to be at their beck and call ALL the time. It's ONE special day that was discussed with the son and girlfriend in advance.
S., I'd suggest that you try very hard not to tie together your son's presence at the birthday party with anything that you and your husband do for him. You do those things out of love and concern for him, not to get anything back. So, don't let it sound like that. Yes, it stinks that he turns his back on his dad who is so good to him, but it's hard to mention that without some people's minds (including possibly your son and his girlfriend) turning it around into some kind of manipulative tactic, when you probably just meant that it was rotten for a son to miss the birthday celebration of such a devoted dad.
Likewise, your son should be present out of love and family ties with his dad. Something else is up if he's saying "he wants to keep his options open" and that the concert is all day. It looks like it's at 7:30 p.m. It seems like coming to a 2:00 party leaves plenty of time to get to the evening concert.
Others have already covered the normal mindset of a 20-year old with a girlfriend. I'd just urge you not to make it into a huge battle, because that will make you and your husband feel rotten on what should be a happy day. However, consider letting your son experience the consequence of making this sort of choice if any of your other children choose to miss any of his special events. Let them. We did this when my son chose to miss a special event of his sister's, with what we thought was an easily change-able conflict (involving a girl). Now, the other two kids have missed a couple of his special events (for flimsy reasons) that I would have ordinarily insisted that they attend (even though they are boring for the brother and sister), and he's back peddling, trying to undo his selfish choice. He wants the family there when it's for him!! (Or, at least, he wants to avoid feeling demeaned by having them choose to miss it for flimsy reasons.) This is working better than the times when we nagged!
Your husband will hopefully have a 60th and 70th birthday, and so on. When your son is 30, he may realize what a little #$%^ he was on his dad's 50th!!