I don't know what age your son is, but here goes:
My adoptive dad and my bio mom split when I was about 8 years old. We had a season of visits for about a year, then mom moved us out of state. I spent nearly 9 months away from Dad, but in the summer got to take an airplane (all by myself! HUGE confidence booster, actually) from Idaho to Oregon and spent the summer with my adoptive dad. I loved it.
A good dad will be a good dad, no matter where he is. I loved getting the one-on-one with him for a month that summer. I loved being around him. Even though he hadn't been on "kid duty" for a long time, he took great care of me. (This was actually a respite from a new, mean stepfather, so this was like a present for me.)
All that to say, I'm sure my mom did fret some before I left. However, things were more than fine. I was allowed to call my mom whenever I needed to. Perhaps setting up a regular time for phone calls once or twice a week-- and then letting your son know that he can call whenever he feels the need-- would be helpful. My mom taught my sister and I how to make collect phone calls, too, and would write down the phone number each and every time in a puzzle book or other book before we left. In the coming years, visiting relatives, I traveled--either alone or with my youngers sister-- without much adult help on Greyhound or taking planes from Oregon to visit family in Honolulu for six weeks at a time.
All was well. Let your son have this experience. He'll learn how it is to 'be' with dad. If he calls and complains, listen and they try not to react, but to be *constructively responsive*. If it were me, I would encourage him to speak up for what he needs *before* speaking directly to the dad, so that you aren't just being 'the heavy'. For better or worse, they'll have to suss out their own relationships with each other. Be available to listen to both of them and their frustrations/problems and offer constructive advice and criticism to both if asked.
I don't know what else to tell you. As for 'if it's going to happen'... this is hard to say, but yeah, I was left disappointed a few times with my bags packed, thinking Dad was coming and getting a call. If this happens, just a lot of empathetic listening is all that can be offered. My mother made the mistake of painting my adoptive dad in a bad light when these things happened-- he was in the National Guard/Army Reserves, and a couple summers, when things were bad, he was called out to fight fires. Our mom, pissed to miss a 'child free' weekend with her new husband, would often make disparaging remarks like "well, I guess it's more important to him to play the hero with all his buddies instead of keep his word to his kids." (She is c.r.a.z.y., by the way. I KNOW you won't do this!) All that to say, yes, it hurt like crazy to have the plan changed, but it would have hurt less if she hadn't thrown him under the bus. AND as an adult, it's been a hard road, but both my sis and I now have a pretty good relationship with him.
Sorry this is so long. All of this is meant to say simply: no situation is perfect, but how you present and handle it-- no matter what happens, can make a positive impact on your son. Even if he ends up being disappointed, your support and love is what he'll remember.