If he's acting normally and isn't losing weight he is probably fine. He is probably getting to full size and his metabolism is slowing so he's not as hungry as in the past. I'd make salads available to him and one bug meal daily and let him take what he needs. Their appetites tend to be cyclical so I'd imagine his appetite will return to some extent after a while. Watch out for lethargic behavior or weight loss which could indicate a problem though.