Love your child no matter what choice he makes. There are ways around enticing children to violate the "rules" from his mom.
For example: A child might feel guilty about accepting a Birthday Present ... or might know that he'll catch hell from his mom if he accepts a Birthday gift or has a piece of Birthday Cake.
No problem .... a week or two early give him gifts just because you love him. Give him a special cake but don't call it a Birthday Cake.
He'll get to enjoy being a kid and won't feel slighted nor guilty. When he gets a little older he will know that it was a birthday present all along and he'll always appreciate your love and affection.
So mom won't let him play soccer? No problem, don't let it upset you, find other fun things to do. Quality time with you and his dad will be more precious moments for him anyway.