I switched from PP Glider Chow to HPW more than a year ago. My babies have gained weight since the switch, but they are not obese.
He must really like the new diet!
He might just need more time to get used to the switch. My babies always eat all their HPW, but often leave some of the fruits and veggies. Some days they don't eat much, I think that is not unusual.
If you are concerned get a digital scale so you can weigh him from time to time. You can leave some PP pellets in the cage for if he needs to snack, or hand feed some treats before he goes to bed.