I would send him the link to this discussion. Your explanation of your reservations is cogent, and he can read for himself that a lot of experienced paddlers back up your position.
IMO, if he wants to take his son paddling, he should sign up with an instructional group, do things right, and get them both some skills. It will cost a bit, though.
Touring Kayak Paddles
Pull-Up Strap Handle Kit
Recreational Kayak Paddle
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