If I were you I wouldn't have any
contact with him at all
especially as he really has hit below the belt by starting a relationship
with your best friend.
You would also think your friend would
not want a relationship with him especially as she knows what he is like
with money and the childen etc etc. But, hey ho there is nowt as queer as
folk.
I would pay for the activities I could afford and send an
email to your ex saying ywhat you can afford to pay for and if your son
wants any more activities your son will have to ask his father direct as
you no longer want any communication with him.
As for your
"friend", jbin her, she is clearly no friend.