at 3.5 i think i'd just mess around with him and have some fun...let him explore....do a littl 'do-re-me" scale. teach him that knuckle song (where you use a fist to play the black keys...know what i'm talking about? if not, let me knowand i'll go into mre detail). i don't think he needs to be getting technical. i mean, maybe some middle C action and learn that going to the left the notes go low, and going to the right they get higher....play off his cues, if he catches on quck and wants to know more then give lessons a go if you want. I don't remember what the books i used when i was little were called, but i'm sure you can find an appropriate one at any book or music store.
HAVE FUN!
HAVE FUN!