Put him on a healthy diet, i would go raw (catnutrition.org can get you started with a recipe). Or at least grain-free canned.
Add to that quality Cod Liver Oil (I get mine from greenpasture.org), Nutritional Yeast (NOT BREWER'S YEAST! get at local healthfood store or online), and Primal Defense Probiotics (Iherb.com has the best price).
The probiotics will help his body become balanced with good bacteria which will get bad bacteria and fungus (including Ringworm) under control. Ringworm can be difficult to get rid of unless you treat it from the inside out by supporting the cat's overall health.
The yeast is high in bioavailable/easy to digest Bvitamins which are great for his skin and coat, and immune system. The CLO is also great for his immune system, coat and skin with very easily digestible Vit A & D in the absolute best form for uptake.
IMHO I would demand the breeder pay the vet bills for the ringworm and see how that goes. I doubt you want to send the poor guy back to live in a situation like that. I've rescued a breeding cat from a home she was miserable in so I know how you feel. Lucky for me she was healthy except for being very underweight and miserable. She LOVES it here =).
The probiotics will work on YOU TOO! Also you can get some Virgin Coconut Oil and eat that, feed some to him as well. I get mine from Tropical Traditions (www.TropicalTraditions.com
). It has natural anti-fungal properties. And you can also use it as a lotion/cream on your sores and his. It will work like a charm if you are eating healthy and so is he. They have a cream i love and i use on myself and my newborn but straight food coconut oil is fine too just make sure you seperate it in a jar so you aren't touching your food stores after dealing with the cat.
Let me know by PM if you need more help. I'm here for you. Or look at my profile and visit my website to get my email address. I'm in the process of writing an article on ringworm treatment and hope to have it up soon but i'm a brand new mom and don't have lots of puter time right now. =)
Blessed be to you and this kitty. He is lucky to have you.