If you build it...dreams will come!
Find a snow glove, thick or thin, that fits your child's hand. This will serve as the foundation.
Borrow a real baseball glove to look at as a pattern.
Search on the internet for "baseball mitt patterns" to find a pattern, or wing it, making your own. It will not be that hard.
Cut strips of foam rubber to wrap around as the fingers. Sew them together, forming a tube, then sew the top shut. Do this for all fingers and the thumb.
Cut strips of leather or the plastic-like fabric to fit around each of these foam rubber tubes. Once they are cut, sew them around it, closing the top.
Sew the fingers together down the center.
Sew an additional strip of leather to act as webbing between the thumb and fingers. This should equal the length of the fingers.
Create the palm and the back, using the same techniques as above. Add more sheets of foam rubber as needed so your child will not get hurt if he catches a ball on the palm.
Sew the "palm/backing" to the fingers. Make to sew together the backs and fronts separately and not together at once, otherwise your son will not be able to get his fingers in.
Put it on and try it out. It may not look like a golden glove, but it will be worth its weight in gold because it was time well spent with your child, in pursuit of their interests and one of America's greatest past times--baseball!