Brian,
 
Our latest rescue was a skin-and-bones ferret, too.  Our vet supplied us
with Eukanuba high-calorie veterinary formula, a canned food made for cats;
she said it was more calorie-dense than anything else on the market.  We had
to force-feed the first few meals -- put some food on your finger or a small
rubber spatula and scrape it off on the roof of the fuzzy's mouth -- but Sam
decided pretty quickly that she liked it, and after that I just put it in a
bowl for her.
 
Keep your high quality dry food available all the time and offer the canned
stuff every 4 hours or so (or as often as you can manage if you're not at
home during the day).  This stuff needs to be refrigerated after opening; we
warm it up in the microwave before serving.
 
If the ferret has a sweet tooth you can also offer Sustacal (found with the
human nutritional supplements at the drugstore) between meals; ours like the
vanilla flavor, warmed to room temperature and served in a medicine dropper
(infant section of the drugstore).
 
Sam has gained 6.5 oz.  on Eukanuba in a few weeks (not bad for a ferret who
weighted 1 lb.  3 oz.  when we picked her up) and her coat is much improved.
Best of luck with your new guy, and please keep us posted.
 
ellen & the Gang of Three (is it dinnertime yet?)
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[Posted in FML issue 2249]