How do you make your dog’s immune system stronger?

My dog has demo mange. I hear it is an immune system problem. I’d like to address the underlying immune system issue without trying to treat symptoms of the issue by drowning him in medicines that can further polute his immune system.

Thanks.

I found a stray dog (mix of llasa apso/poodle/terrier!!!) in a rubbish bin,  almost 1,5 years ago. He was in a terrible shape, no hair, and stank to high heaven. The first vet I took him to, diagnosed mange (demodex) so I did the treatment (dips) for 12 weeks, and then he tested negative.
But 2 months ago I tried a different vet, as I wasn’t so happy with the other one. He found that the dog has Leishmaniasis, so I am treating with Allopurinol.
He also has mange again, which apparently is because his immune system is not working properly.
He also has infected paws, the antibiotics do not seem to be improving the condition much.
I don’t think he was weaned by his mum, which is why he has all these problems, so my question is:
Does anyone have a similar experience, and can you suggest something to boost his immune system? He is a lovely little dog, and is very happy, but I would like to find a cure for him.