DPs ex has systematically been organising more and more activities for DSS that coincide with contact time with his Dad - with no consultation with DP, and most recently, not even notifying him.
DP already has a commitment on that afternoon (which he currently fulfils with DSS) so he won't be able to collect DSS afterwards. It is reasonable to assume that DSS mum will be collecting DSS after the club and that DP can pick him up later on?