He could still be hungry, have you had him long enough to feel with certainty that this is unusual behavior for him? Could it be attributed to him still settling in to a new place (since you ref'd to previous owner)?

If it was me, and I was planning to switch to rats anyway (easy for some, drama bomb for others), I would just wait until the scheduled rat time, no extra mouse. He's not going to starve in that amount of time and being a bit hungrier may help entice him to switch over. Plus if it's something else that's stressing him out, he can have a chance to adjust without having more stress on his body (from digestion).