One of my fuzzers, Simon, has a weight problem but it fluctuates. He'll get pretty plump and then slim back down to normal. I'm not sure what the "normal" weight for a ferret is offhand but I would try cutting down on the treats and ferretvite and see if that helps. If he starts looking obese I would ask a vet.
I don't have a small animal scale but I'll look for a used one on craigslist or something. I'll take a pic as soon as I can. I've already cut back on the treats but its hard when he comes up and stands on his back feet with his front paws on my leg- such a little begger boy. I can hear him now... "mommy.... treats, please..." Teddy isn't as interested in treats as Kodi is.
Yes cut out the treats and the Ferretvite. Ferretvite is really used for sick ferrets or ferrets who are underweight. It's really high in calories. Ferrets also don't need treats so either get rid of those all together or limit it to one a day.
Does Simon only do this at certain times of the year? Ferrets often lose weight and their heavy coat in the summer and then put weight back on and get a thicker coat in the winter. It's a natural part of helping them with their body temperature.