Always offer him at least one or two more pieces as well. If he is still hungry he will eat more. Night Crawlers are easily digested and pose little to no risk of impaction from filling your frog up. As he grows you can offer whole worms. Start with smaller sizes and feed larger worms accordingly.
If you'd like offer him 1 three to four inch piece. This way he will get used to larger whole worms when he is able to eat them. With my frogs I just give them how ever many they want and if I feel I need to cut back I will. Most are really good at regulating their food intake so you should be fine.