How small is your grandson? Put him in a life vest and go out in the
canoe (not fishing) just to let him get used to the feel of it. In a
small pond or lake, let him splash and even tip it over, so he can
learn that even if flipped it will not sink. He can learn not to be
afraid of the water.

Also, if really concerned, you can get removable outriggers for a
canoe.

However, if neither of these seems an option, perhaps he really is too
young and you guys should just plan to fish from the bank for a year or
two.

A canoe or a yak is really a great way to fish. I also have a
porta-boat (14 foot) folding boat with 9.8 hp motor.

If I had my druthers I'd use my yak (a Tri-Balance with outriggers) all
the time as it allows quiet approach that a motor boat can never equal.
Seeing the wildlife up close makes every day on the water worthwhile,
even if the fish are not cooperating.

Frank
North Florida