I don't know if it's too late, but I get size 80 for my 13 month old. He is not huge, but not small either, about the size of your son. The size 70's do still fit him(and get worn), but the 80's are a better fit at this point IMO. The 90's are HUGE still and are not ready to wear, really huge. This is one company that really doesn't need to be sized up, excellent quality clothing.