You need to consult with a family or matrimonial attorney, who can review the terms of the agreement for you, for any sort of a definitive answer. What may be critical is how the agreement is constructed as the  "agree on" or "agree on it" means. If it means that you can, at will, decide whether to pay for any given sport, or indeed sports at all, that'd be one thing and you could probably stop paying. If instead it means that you get to decide jointly with your ex-wife which sports you son will play, but once that initial agreement is made--once you've agreed on a sport--you are bound to keep paying for it, you'd still be obligated. Someone needs to review the entire agreement with and for you, to get a sense for what it means in this context. Good luck