how big did you buy them? did they not come with braces? try debenhams or a dress shop they sell braces not percific to skiing but would work, although could be fun watching him ski with his trousers round his ankels
dont know other option if you go into a dress altering shop may be able to fit them with an elastic I have had this done for my daughter with jeans as she has a simlar problem, will need to take him with you and they just put two small bits of elastic on each side and it helps pull them in. possibly about the same cost as braces