My 7yr old son told his mom his "weewee got hard"... No clue how to handle it.

Dad here and a throwaway because I don't know how to feel about posting this here but I couldn't think of any better place either. Weird, I know.Anyway, I picked my boys up from my ex-wife yesterday evening for our weekend together. She pulls me off to the side and tells me that after the boys watched a movie (the new Jungle Book) my oldest came to her and said that during a certain part of the movie his "weewee got really hard and he didn't know why". My ex went to do fold laundry and he restarted the movie and fast forwarded to the part where he got hard earlier. She snuck up behind him and asked him if it happened again. Ashamedly, he said yes.She has asked me to talk to him about this since this is a daddy issue (I agree), but I don't know how to bring it up without embarrassing him to death and I don't know what to say.Any advice would be great! Thanks! via /r/Parenting http://ift.tt/2gTkkB3