If he has a schedule, honestly I would keep him on it, but bump him up to the right size meal.<he won't starve to death I promise but might be a bit testy until then.> I'm 'guessing', judging by the pic, he's somewhere between 160-200 gr so you can use the chart posted earlier in the thread to decide what to get him. He can easily do a large adult mouse or a small weaned rat. Should range between 25-30 grams if you use the 15% rule.
First thing though... get a digital scale. It's gonna help you a LOT as your baby grows.