Okay, now I'm hoping I'm not wrong with any of this, but I'll try to help as best as I can.
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Ideally, with him eating so many different foods, there shouldn't be a problem with vitamins/minerals.
If you want to keep him on a prey diet, then he'd be getting probably the best mix. Different aged mice and rats, rabbits, baby chicks and quail, are all pretty much nature's perfect nutrition package. Though, I don't know too much about feeding a prey diet, heh...
With a prey-model raw diet, things are a little different. He would be getting most, and ideally all of the nutrients/vitamins/minerals/etc. that he needs from the diet.
He should get one regular sized chicken liver, or a piece of beef/pork/lamb liver of the same size, one time per week. Liver has Vit. A, Iron... Zinc, I think... and some other things that I can't remember right now. xD He should also get another organ or two (Kidney, spleen, pancreas), roughly the same amount as the liver, once a week.
Dark meats off of chicken and turkey (legs, thighs), red meats, and hearts contain Taurine, which is essential for ferret health. Ferret's can't store taurine in their body, they use it when it's available and that's that. If you're feeding dark meat turkey/chicken, red meats, and chicken hearts regularly then there shouldn't be a problem with it.
I can't get my kids to eat hearts all the time, and one of my girls, Tikki, doesn't like red meats too much, so I supplement Taurine in their diet.
Another thing that is good for them is fish body oil, it contains Omega-3. I get salmon oil caplets and the kids get one a couple times a week. However, if you can get him to eat fish like sardines, mackerel, etc, for a meal once a week he'll be getting it from them. I think a meal of fish once a week should be enough.
I'd suggest getting him to try some red meats, beef and veal (veal is popular with my kids), maybe some lamb. If you have a place that you can get rabbit from, that would be good too. The more variety he gets, the better for him.
With weight... Well, right now you said he was... four months? That's pretty young, and he's chowing down with all the hunger a kit can have. The young ones eat like crazy, it's almost scary just how much they eat. xD He's probably going to be weighing more in the next few months, especially now, not only because he's still young and in his growing stages, but because winter is coming and he might just be packing away winter weight as well. I got my Mis at the end of April, he was 2 lbs at around 5 months old, and he's just over 3lbs now. He's starting to gain again now that the weather is getting colder.
I wouldn't worry about Kirby getting overweight. He probably won't settle into his "normal" weight until the spring now, and he might get a bit chubby over the winter. If he's getting enough exercise I don't think weight should be a problem.
I'm not sure about the poop and urine stink... For my guys it really depends on what they've eaten. If they've gotten salmon oil earlier, then their poops are a little fishy. xD Usually, the smell isn't that apparent, unless they've just gone.
Hope that helps! I'm not an expert, and I'm sure more knowledgeable people will come along.