We had a few runts in a clutch from time to time, many survived and actual attained average sizes. Rat pinks every five days for the little guy, hit him up to rat fuzzies every 5 days at ~60g , then straight to pups every 6-7 days. His behavior makes me think he is thermoregulating or has just gotten used to the place well enough that he wants to move about.
What are you weighing him with? I am kind of doubting it right now.
We Purchased a mechanical scale at wal-mart :\ It was the most inexpensive one they had.