Feline immundeficiency virus infection does not lead to acquired immunodeficiency syndrome in cats as often as human immunodeficiency virus leads to AIDS in people. Feline leukemia virus infection in a cat that persistently tests positive is more of a problem. This cat should be kept separate from your kitten, in particular, and all the other cats, in general. If these cats are all the cats in the household you will just have to be aggressive about seeking treatment for illnesses that crop up and try to keep the stress levels as low as possible. If you have other cats the choices are much harder as these are contagious illnesses.
Many people do manage multi-cat households in which they keep infected and uninfected cats separate.
It is hard but it is possible. I am sorry to take so long replying to your note. A good quality cat food should be sufficient. It would be a good idea to have your veterinarian examine your cat if the weight loss continues. Cats with FIV also get diabetes, kidney failure, hyperthyroidism, liver disease and other cause of weight loss.