I just have to share this ... yesterday I was at a schooling show and we were waiting for my classes and there was a stud colt in a halter class that saw something he liked and his down stairs friend came alert and he was dancing all around and that friend was flopping all over .. yes we laughed like no other but I think he realized that is was uncomfortable to have it wacking all over and he stopped ... totally random I know but we got a kick out of it!
Mine drops when he's being brushed and it's warm and sunny. Relaxation. If I'm riding and he does it, he usually has to pee.
I'd just work him, unless he does have to pee...which it doesn't sound like that's his problem.
Hah, this may sound kind of harsh, but I know of one lady who shows a lot of young stud colts. When they get . . . Excited . . . She gives them a little whack. They learn to keep it in when they are working.
PLEASE don't hit is penis.... How would you like to be slapped with something on your vagina or penis every time you were relaxed???????? Every time he drops, make him work. You can INJURE his penis by hitting him!!!!!! Especially the tip of his penis. The tip of any animal's penis or the clitoris is EXTREMELY sensitive. When you clean his sheath, teach him to "drop it" and "hide it." Eventually he will learn to put it away.