Hold up on canceling the whole thing. This gift was for your son, right? Because he loves this team. If his dad wants to turn it into a pity-party because you won't be playing Happy Family then cut him out of the equation and let your brother take DS. If you cancel everything Big M has managed to control the (now lack of) event with his drama.
Wait and see what happens. He'll either write back "Okay, I'd love to take him. I'll get my ticket." or "On second thought I can't make it. Blah, blah, blah, whine, drama..." in which case you say "Okay, I'll let Brother know so he can make arrangements to take Son."