One study theorized that this is a stage among children (children tend to compare their sexual organs etc.) But what you witnessed is beyond normal. He must have seen it somewhere (because kids do not listen to what adults say, they imitate what adults do). Talk to his parents about it.
Also, I personally do not encourage you to kiss his penis at his age kahit pa lambingan sya. It would seem to him na normal lang ang ganung behaviour. Teach kids, regardless of sex, na it is not OKAY for anyone to touch their genitals except when you are going to wash it.
Magbasa pa
✌️