My 10 month old isn't gaining weight fast enought 4 his doc so he told me 2 give him pediasure & now he has watery stood could the pediasure causing?

It is possible... It is possible that his watery stools are just a coincidence. You could try cutting back to one can and see if his stooling improves or stop altogether for a little while. Once his stooling goes back to normal, you could start things up again slowly. If things brings on the watery stools, you may need to try to find a different nutritional supplementation to help him gain weight.