Isn't fred a new pet? I've heard ferrets can gain 1/3 of their body weight in fat during fall, and they will loose it again in spring. The average weight of a ferret is 1-5 lbs, as your boy weighs 5 lbs now I"d be careful to make sure he doesn't get much heavier :P
If you find that hes gaining too much weight too fast then you could give only a few cups a day or something like that. My two only get 2 cups a day and by the end of the day they still have some left so I know that they aren't starving
I think it also depends on the type of ferret. I had a silver mitt and he was much larger than my cinnamons and albino. My albino was about 2lbs and my cinnamons were 2-3lbs. My silver mitt was close to 4lbs. The other silver mitts i had seen previously was also kind of big.
Cute picture. That one looks like a cinnamon. If he was lite brown to reddish as a baby then its a cinnamon. Mine were about 2-3lbs, year round. There weight didn't really fluctuate that much. My male was a little heavier than my female though.