I dont think that he will be destined for a miniature size... he should catch up shortly . i know mine grow more in the summerthan in the winter due to constant grazing in his outdoor pen.. he will still grow in the winter but just less....i live in newengland and my torts are outside/greenhouse about 6 months out of the year. my sulcata started off slow too due to the wrong set up but now like you i have corrected the problem and hes doing fantastic. hope this helps