Most ratties will adjust their nap schedules around their humans. My boys are up at around 4-6:30am since I'm up too around that time; then they sleep for hours, getting up for 30 mins or so when I get home from work, then nap some more and are full of energy from 9pm until the early morning hours. It could be that he snoozes while you're away and is more active later in the evening. Every rat is different. Rats sleep on average for 18 hours in a 24 hour period, so it could be that he is awake periodically throughout the day and that may change as he adapts to your schedule.