I'm not sure how old your son is, and your approach would have to be adjusted based on his understanding and maturity level.
I think, overall, it's essential that your son understand that is not a reflection on him, his worth or his lovability. His father is not rejecting him - his father has some problems that prevent him from being attentive all the time. Don't scare your son by saying that your ex is sick - but you can say that his father doesn't always do the right thing or isn't always able to behave the way your son would want. Don't give your son the idea that misbehaving can be excused all the time by a disorder - that gives him carte blanche to do the wrong thing with you! Tell him that his father loves him very much and that sometimes there are difficulties in staying in touch. Sometimes that's technological, and that sometimes it's due to the distance or the time changes (even if that's a stretch, it's possible or at least was when he was farther away, which he may be again, for all you know).
Perhaps you should re-think the family trip - I'm not sure if that's extended family or just the 3 of you, but it puts an awful lot of value on a get-together that may not happen. Also, if your ex is so erratic, maybe having him along is not a good idea. Is it possible that your ex is stressed out about this trip and is putting up barriers now?
I don't think you can keep your son from being disappointed - he is, and you are. I think feeling unimportant is much worse, and it comes from feeling that his father's actions are the result of a little boy's actions. They aren't. His father is a grown-up, and sometimes grown-ups don't do the right thing. And if your ex is suffering from a disorder, and if he's erratic regarding girlfriends, and if he's attempted suicide, he needs to get significant help. I'm not sure you want your son to spend a vacation with someone so erratic who lies all the time.
Talk to your children's library staff about some helpful books to read when a parent is mentally ill. It isn't important that you name the condition - I don't think that's information a young child needs anyway - but understanding that there are things that cause people to engage in behaviors they don't wish to engage in can help explain things. Just make sure he doesn't think it's going to be something he inherits from his dad.
Good luck.