I think GAD work best on chubby thighs, so if he's at the small end of the weight limit they're probably too big. Is the insert saturated?
Mom to Reya (13) and Little baby Lila 9/22
GAD larges are for a 3 year old, IME. My ds2 was 25 lbs and 2.5 years old before they fit him - tall and thin.