We are no longer recommending the oleander supplement for pets, as their digestive tracts differ from humans and they tend to get major stomach upset, so much so that it is not worth it. Oleander has worked great on pets when it has been injected, bypassing the digestive system. Right now the manufacturer is working on suppositories that will bypass the digestive system and be more effective, but it's not ready yet.
I do want to point out that the pale gums could very well be anemia and this could be life threatening. He does need raw liver daily, but keep an eye on him, as I don't know how far it may have progressed. I don't want to alarm you, but be ready to take him to a vet.
For hypothyroidism, you may want to try a product called Thyroid Soothe by http://www.petalive.com. And I want to point out that I have no personal affiliation with them. However, I use their products on a regular basis and find them to be outstanding. If it was my little Heidi, I would certainly give it to her.
One thing I would suggest though is colloidal silver in his water daily. This could be effective in treating FIV and will help strengthen his immune system.