Hi,
The other ladies are right regarding the probiotic. Nystacid syrup that they swallow because my paed said that quite often with severe cases of thrush it travels through the digestive tract where it can stay for a long time until treated properly. Once my lg, had the drops to swallow and then the dactarin for the mouth, which always worked really well.
Make sure everything that he puts in his mouth is boiled.
As for the immune booster .... ah that a question quite a few of us mommies would pay a fair amount for.
I tried the creche guard with my daughter who suffered terribly with her throat and ears, didn't work for me. I tried a few homeopaths , had the echinacea and lots of other little dark bottles, but they didn't help. I guess also your child, well my daughter got to the point of no return and the only other option was tonsils and adenoids out. Since then, as the saying goes quite often after a tonsillectomy, she was a completely different child. No more antibiotics.
My daughters case is completely different to yours, so maybe a homeopath could help. I would give it a go.
Also stay away from sugars - easier said than done.