E.T.
this is just my opinion, so take what you want and leave the rest...
You can't make him be a dad.
You can't make him put his child first.
You can't make him exercise visitation.
All you CAN do is make sure dad remains in a positive light and you make your son available for visitation if your ex decides to exercise it.
You're in a bad position. You're always wondering if he's going to come or not, what's going on, and why he's not thinking of his son and wanting to spend time with him.
I understand your son may not want to go with dad, but not much you can do about that either. If your ex wants him, your son has to go. It's not an option. That's his dad. Some things are optional, some aren't. Hard to explain that to a 6 year old though.
In my opinion, I would stop putting in so much effort to try to make your ex be a dad. You're making yourself crazy. I would back off a bit and let your ex come to you. Make sure your son is available during his appointed visitation times, but don't make it seem like you guys are standing around waiting for him to call or come by (so your son doesn't pick up on this).
It's not your job to FIX things b/t dad and son. I think it IS your job to make sure your son knows his dad loves him, but you can't force your ex to do something he clearly doesn't want to do, which is step up and be a real father... He either will or he won't and I doubt your efforts will sway him one way or the other...