I'm going to buy my son a gift
Why are you buying him a gift?
I love him and want to see him smile.
It can be a great joy to treat your child with a gift. It is also very nice to make someone else happy.
I want to also  keep him from ratting me out to his mom in the future.
Even though it's for the wrong reasons, it's still nice of you to buy him a gift. Though I hope you do actually love your son.