first thing: get him a stool to stand on if you expect him to lift the seat and stand to pee. having a heavy toilet seat crush your penis is not a pleasant experience so he will be a bit overly-cautious for a while. as long as he is urinating okay (no blood. no difficulty staring his stream) the penis is likely fine. of course i can't see it so you need to be the judge whether he sees his medical provider or not. it could have been worse. like getting slammed in a door. toilet seats hurt but they are not that heavy. cold compresses could help but who really wants a cold weenie! in most cases just leave it alone and make sure it is healing properly.
how do you treat a toddler's bruised penis? bruised due to trauma dropping of toilet seat on it