honestly? sounds like classic celiac to me. It still does include healing the gut, but the FIRST thing I would do immediately is to take him off of wheat/wheat gluten, cow's milk, and oats, and I have a feeling you'll see an improvement. my son has celiac's disease. We are now working on healing his gut. I don't ever expect to see him able to eat wheat/wheat gluten/dairy, but his attitude/behavior and overall demeanor has improved soooooo much that it's like night and day. He is five. Every once in a while his sister will sneak him part of her bagel and I can tell INSTANTLY. His behavior goes nuts, he breaks out in eczema in all his telltale places, and his poop goes to water. *sigh* The antibiotics he has been on can DEFINATELY have caused a "leaky gut" which could then have caused him to "get" food allergies that he otherwise could have avoided. THIS happened to my husband, who now as an adult has multitudinous allergies that he never had as a kid because of abx after two surgeries in the last five years where they gave him broad spectrum drugs to keep his gut "healthy" when they cut into it.
Check out the allergies forum down in health and healing, here on mdc. I'd be glad to give you all the info we've gathered over the years as far as avoiding foods and using other things, etc. It becomes a new way of life.
EVEN if it is yeast, I would say to stop all sugar, all grains for a while. There are several things you can do to heal his gut, and get rid of the yeast, but you WILL want to see a holistic/naturopath doc so that he gets pediatric doses. Otherwise, you'll want to do a lot of research, and perhaps can address it on your own, as far as doses go. Marshmallow root heals mucosa. Lots of probiotics. B vitamins in a complex once a day and straight biotin once a day can keep the yeast from multiplying. Things like osha (uuuggggh, it tasts so terrible that I just canNOT give it to my kids...I have a stomach of steel and it makes me gag for an hour after I take it), black walnut hull, and other herbs can help, but again, you would want to be VERY careful about doseage and use, and a naturopath could help you there. I have heard that some people will actually use goldenseal to KILL the yeast and then immediately go to marshmallow root and probiotics with biotin to stop it from coming back and then replace the healthy bacteria. Garlic, astragalus, reishi mushrooms can all be found in kyolic immune support blend and are all super anti-yeast. HOWEVER. Any of that stuff has to be done for at least a couple of months at a time to be effective, and you have to STICK TO IT. Oftentimes you might find better effect from going with a seven day course of diflucan with a ped and then going back to the kyolic/marshmallow/probiotics/biotin to keep it away...
Honestly, the best course is to REALLY educate yourself and then go in with a lot of knowledge so that you can have a conversation...and expect that a mainstream doc will only have diflucan to offer, and absolutely none or very litle experience with celiac. A naturopath will have more experience and help as far as yeast and allergies and healing the gut. None I know of take insurance. Many have initial fees in the hundreds of dollars, and then visits after that are much more reasonable.