Even at 4 months old a veiled should still be putting down a lot of food. How much is it eating now? IMO he shouldnt be slowing down the eating at this point. Do you offer other feeders other then crickets (maybe getting bored with crix)? Have you offered larger crickets? If so, that may give you the impression that he is eating less due to the larger cricket size.
If hes that big i would be giving him the largest crickets you can find. Its silly to spend money on small food when he can handle the big stuff. When they are growing they will eat as many as 20 a day.