some good advice here.....I've been through it with 2 kids ..both starting at the age of 4..One could ride a bike w/o training wheels ,,so he started moto w/o training wheel .The other started with traing wheels ...Both on PW's..
Get him to ride bike w/o training wheels first ..That balance is so important ..And riding a dirtbike with training wheels ,,they learn BAD habits ,that are hard to break when the trainers are removed...
And as someone said ,,those little KTM's are rocket ships and not a good learn to ride bike,,,one whiskey throttle with a gnarly wad-up and they will be afraid of the machine from that point on...I would guess only about 5% of little ones have the no fear balls out ,pain tolerance to learn on the little KTM's.
Get him on the pw,,let him build confidence and then move up,as quickly as you can > You will never lose money on a PW.
And remember ,,in a few years ,when he's on KTM jr's and Sr's,,,he may look and act like a big boy when he's on the bike and kicking ass,,but under that helmet ,,is still the mind of a 7 yr old.
Good luck man ,,and just enjoy it