I'm thinking of potentially ruining my boyfriend's relationship with his mother
Why would you ever want to ruin a mother and son's relationship at all?
I see that if he's not closer to his mother then I'll have a better chance to win him
It's understandable to want to spend more time with him, but its evil to want to ruin a parent and child's relationship in order to force someone to become closer to you. You'll come off looking manipulative and controlling if he were to ever find out what you did.
He'll never find out.
If you ruin your boyfriend's relationship with his mom odds are you are going to worsen your relationship with her. You need to make the relationship with your boyfriends mom a good one for you and him if you want to last with him.