This patient support community is for discussions relating to the challenges of parenting
toddlers (age 1-5), including physical, speech, sensory, cognitive and emotional development, choosing a daycare/nanny, games & activities, and toilet training.
All you need to do for your son is tell him it's extremely rude to do that (and maybe in this case you could say "dangerous" or "unhygienic"). just...treat it as a misdeed like any other.
I'm a mother of a 4 year old boy and a five year old boy---- know lots of kids and this is not normal exploration. A poke and a giggle I could buy---- but not what you describe. And if it continues, other parents that are told of this will complain. It is a documented fact that kids that have been molested go on to molest other children. Not okay. Please seek help for your child. Good luck.
Normal exploration for his age would be to want to look, and possibly touch, other's private parts - usually girls parts.
For him to get on his knees and try to lick another boy's penis is learned behavior, in my opinion.