I think that maybe you could say something like has your dr. said anything to you about how big your son is growing at his age? And just say that you know that your son and him are good friends and that you dont want to ruin their friendship by asking you this but, have you ever thought that maybe something else is going on with your son like maybe giantism? I know it is hard to say something to somone about their child and you not knowing everything that is going on and so forth but you never know his father might be thinking the same thing you are about his height and is just scared to say something about it. But a question is never a dumb question. I would just explain to him that you were just looking it up on teh internet one day and you were thinking about his son and his charcteristics. It is not like it is a real bad thing right? I dont know much about it. But just say I am sorry if i have stepped over the boundries and if i have offended you in anyway I am sorry.  But like i said you never know his father might be thinking the same thing as you are. You never know unless you ask. Right. I wish you luck.