I too spend most of my life on family get-togethers (just me and my parents, whom I live with) and hobbies. We take a trip once a year. If I had to postpone a trip, I'd flip - even if it's for a similar reason. I actually had to do it once, for a week - and it drove me totally insane (I was still able to go, so it feels much worse when someone else can't). I ended up acting very selfish in this situation (and another one, where we ALMOST postponed) and I felt very guilty afterwards - in both cases. Still, it drives me insane since I'm a very plan-oriented person, very averse to sudden changes - plus I love taking trips just to get away from the vicious everydayness of the vicious everyday. I'm really not a happy person within the mundane reality.
Make sure to be supportive towards your friend of course, setting these feelings temporarily aside when you're with him. As far as the trip goes - I guess maybe go by yourself, or with someone else if possible, this year but go someplace else (since this specific trip is supposed to be for the two of you, thus making it a special friend trip), and then save this originally planned trip for a time at which your friend might be able to go? I'm sure your friend wouldn't want you to miss out on something you really want to do - and what you want to do is to just take a trip, right? So yeah - take a different trip this year and save this "special trip" for the two of you for when he'd be able to take it, and I presume no one knows that yet. Even if you take this other trip without him, you'll still be his friend and you'll still be supportive of him.
Unless your specific goal is to only have a trip with your friend, not on your own or anyone else - in which case the only thing to do would be to just wait and hope; you'll be a very good friend to him if you do this.
I'm thinking about taking a trip with my folks later this year and I'd go bonkers if it gets postponed (or...Dear God, I cannot say that word, ca...ca...) - rescheduling really is a pain, as well as the fact that I'm extremely bad at handling sudden changes - unless they're good changes. As last resort, I'll go by myself because I refuse to not go at all. There's no ca...you know. I am very persistent in the pursuit of happiness because I really need more happiness. I'm not a happy person most of the time and these trips...they bring out this sunshine that's always been hidden within me, they make me someone who's pleasant to be around! They make my anxieties nearly absent (some fleeting minimal ones might surface but they're negligible). I absolutely cannot miss out on that. Any trip-related change would just send me into a complete frenzy - or regarding just about anything that I ever look forward to.