Good to hear he is doing better! You might consider picking up a bag of Purina Game bird starter (30% protein) and just add a bit to the regular feed for a month or so. Unused starter can be stored for several months in airtight, plastic 3-5 gallon containers. We always keep some on hand to...
You're absolutely right (poultry/humans/equines same-same). Should probably start with Wazine in the water, just to be sure. However, considering the description of the poor guy's previous environment I'd expect a heavy worm load would have resulted in no picture of Jack being posted...
Turkeys are very much creatures of habit. Change, even for the better, requires quite a bit of time. Definitely check feet/hock/breast. If cage/igloo wasn't cleaned regularly I'd suspect foot pad dermatitis and hock burn (breast blisters might also be possible) resulting from long term...