Howdy, Shakes.
When I was growing up, I was the kid who didn’t go with Dad on some weekends. It’s fairly normal, but if he wants to skip a 4th weekend, I think you should have a talk with him about it and see if there are other issues, or if it’s just not wanting to miss out on stuff with his friends.
Depending on his age, yeah, it’s prolly just his time to flex a bit and start seeing what life is like. Early teens? Gonna happen.
It did with me.
But there was also an undercurrent, a tension, in the relationship between my father and I that ran strong, and because of it (my learning that my father was a liar and a self-centered asshat of staggering proportions), I also was using the real opportunities to spend time with my friends to stop seeing my father.
It took us years to work thru the issues between us, and today I am much happier because of it. I don’t know how my dad feels about it all, as I haven’t spoken to him in over 20 years.
Anyway, my point is that I hope you and your son have a good relationship, that he is merely getting older and exploring the avenues he & his friends have access to, but since you seem to be a concerned dad, you should talk about it with him and make sure you both understand what’s happening.
Ask him what he does with his friends, ask enough questions to be knowledgable about your son’s activities, not just familiar. And don’t be afraid to tell him you miss him when he doesn’t come, to see you, but ffs don’t allow it to become a guilt trip.
Just be honest and upfront, and genuine, and all will be well.
Good luck!
ETA: FWIW, I was absolutely devastated the first time I had to go out to the car and tell my old man I wasn’t coming. My Mom made me do it (she wasn’t going to shoulder any blame or flame for what was solely my decision), and I did, but I felt like a piece of shit for quite a while afterward (days, not hours). I’m fairly sure your son didn’t just deny you and then breeze out with his buds, eh. I’d be willing to wager it took it’s toll on him, too.
EATA: I like statsman’s idea as well, and Girl’s idea of maybe season tix or something so that you and your son have scheduled time together that’s not just “hanging out on weekends at Dad’s”.