I hear you. I haven't been able to keep long and deep relationships with friends like you as my locations have been so fragmented, but I have been observing the same about a particular friend. She hasn't returned quite a few of my calls. She's pretty busy as I can tell from the publishing in journals and such, but I still wonder sometimes, and then I let it go. Whatever will be, will be. I also had a friend rant about being the single guy among a sea of paired, housed, and no longer child-free couples and feeling left out. (This probably applies more to me than you.)
All you can do is give it time. Who knows where he is in his life, if even he does? Seems like he himself is going through a lot of changes.