Although this is normal, you need to tell your son that this is something to be done in private and not make a big deal of it. At this age it's like their own little "worry stone."
I raised two very healthy boys but they weren't humping everything in sight, what they did in the privavcy of their rooms, I don't know. It does kind of concern me that he was laying on top of a teddy bear doing this, I feel he had to have seen it done this way somewhere and not in a cartoon. Don't tease him about it either, it's something that is natural but done in appropriate places like their rooms. I don't think I would say anything until you see him doing this, and then ask him where he saw this and then talk to him about it. Just stay calm when talking to him.
I agree with leighlay. I'm a single father of 3 boys under 7 and raising my lil bro 13. What ur son was doing is natural. He doesn't need to see it. My sons are always playing with themselves. My 6 year old humps things. Boys will be boys! If it feels good, they are going to do it. Again it is totally normal. I wouldn't worry about it. He is just being a typical boy, and it gets much worse the older he gets. U can tell him all u want that playing with himself is innapropriate, but little boys don't really care. They do it without knowing sometimes. If u don't want him doing it, keep him occupied by other things for instance, u see him doing it, go give him a toy or let him color, etc. Hope this helps. If u have any other questions feel free to message me.
I have a 5year old little brother and i always catch him feeling on himself, its only natural for a boy to learn they have a penis and when they touch it , it feels good. when i ask him he just blushes and rubs off the question and walks away. If you don't want him to do it, you should tell him not to touch himself, but at his age he's just learning. you can ask him about it or why but i doubt at 5 he can really answer you. Or ask if he saw it somewhere.
i would tell you to observe him but dont let him know that. may be he was fighting with the teddy bear or the babysitter saw was wrong it can happen. Are you seeing any different behavior of your son . dont let him see the cartoon which you think that he learned that thing from that. hope this help you .