I'm not sure where you're going with this message

First thing first, there's no way saying to someone (either a boy or a girl) that people would "get wet from seeing [them]" if they would loose weight is okay. Now, I think that people who thinks Sean is sexy as he is right now wouldn't think anything less if he would loose any weight, not even a pound! If he feels good as he is right now, it's not up to you or anybody to say him otherwise. If he wanted to loose weight, it would certainly be for himself, if he thought he would need to do so... not because some internet stranger told him that he needs to! You probably meant well with your message, but all in all, I just find that kind of things rude...