How to get my 4 year old to stop

How to get my four year old to stop peeing in his underwear!!!! Ugh lol. He doesn’t pee like all of it, but he does pee a little, I think he forgets and starts peeing then stops himself when he realizes. But he goes through so much underwear every day and I don’t want to put pull-ups on him because they can get expensive & because I obviously want him to be a big boy. Help