That's ring worm - why hasn't your vet put your baby on an Anti-fungal??? There are also special baths you could give and topical treatments. The ongoing scratching combined w/ scabs etc. could eventually lead to a staph infection - so I would treat asap.
For the diet, are you balancing the cal-phos and everything else? That's extremely important; it can be dangerous if not balanced properly - if cooking longer-term. If it's not currently balanced, the immune system will be affected by this imbalance. The best way to "boost" the immune system is to provide the best nutrition possible, with all the needed nutrients, and perhaps to add things in like Omegas, digestive enzymes, probiotics.
Which dry food are you feeding?
I am so sorry your baby is suffering like this - I know how terrible it is to watch them hurt.
