Ideally his dad would take him on those weekends and your son would enjoy the sport to it's full potential. But if you can't agree there is no harm in a small compromise so that perhaps your son only attends every other week when he is with you.
At least he is getting to play in some capacity. It's not great - especially being a team sport. But it's def better than missing out all together.
You can lead a horse to water.. But you can't force it to drink!!. You can only do so much when ultimately his dad gets to decide what happens on his weekends. Eventually your son will see that his dad isn't a team player and is letting his son down by being stubborn.