Well, then maybe see if he will do once a month on a long weekend from school?
Sounds like he really has some growing up to do. Email him back and forth for proof that you tried. If he still wont come, then email him to call you a week before a weekend he can do, and you will try your best to have child available. Keep the email going with school and activities schedule. Every now ant then you can send him a copy of
report cards or whatever in a registered letter to keep him up to date and have proof. Keep texts or take pictures of the phone when he cancels or says no.
If he wont do anything, text him the Wed before his weekends and ask if he is coming that weekend. Then keep it. Start telling child that dad is working and he will not go there as ofter but loves him. You will let him know when he is coming.
Is it a money thing? Maybe offer him some gas money to help get son back and forth. Really not your issue but if it gets your son to see his dad?