It does sound as if he's had either an intolerance or an allergy to a food. It can take anywhere between a few weeks to a couple of months for the gut to repair itself and in the meantime it would be better to aim for good digestion rather than try to pack weight on him ;)
If he's looking so much better in the last month then I'd stick with the JWB and the 4 meals a day, I'd also stick to the same amounts, he will gradually put the weight on - it sounds like he already has ;) Once you're happy with his weight, then you could try gradually reducing the amount to that suggested on the packet.
James Wellbeloved also do a lovely range of treats, Crackerjacks, Pure Incentives, and light treats which I'm sure your dog would enjoy as a training treat ;)
http://www.wellbeloved.com/products/dog_food/pure_incentives.aspxIn your position I'd stick to what works ;)