Take him to the doctor duh

That reply was a little rude I think.
My 1 1/2 yr old gets bumps on her bum sometimes too. It all depends what kind of diapers you get to change, if they're stinky it could just be an irritation. Give it a few days and try putting corn starch in his diaper(it absorbs the moisture) it works honest especially for a diaper rash. Anyway try that and maybe some butt cream and if it's still there in a few days you could take him in. If he doesn't seem bothered by it or unusually cranky I wouldn't worry too much about it