A good multivitamin for my cat with a weak immune system?

My cat is almost a year old, and he is an active carrier of coronavirus =( unfortunatly when he was three months old his sister developed FIP. Cheshire is active, but I always worry that maybe he will get sick and I won't be able to help him. I really would like to know if there's anything specific I can do for him to boost his immune system. I've read online vitamin c and l-lysine are good supplements. Is there maybe a good multivitamin I can give him?

L-lysine is known as a treatment for feline herpes virus - I don't know whether it would do any good against coronavirus, but I doubt it would hurt anything. Stick with the powder kind - the gel sometimes has artificial sweeteners in it, and that can cause allergic reactions in cats.I would look online for holistic veterinarians in your area and call one for advice - when it comes to strengthening immune systems, that is their primary form of treatment so they usually have lots of experience with supplements and many have a store you can buy them from at their office (or they can tell you where to get whatever it is). Avoid giving anything made for humans or dogs unless a vet has given you directions to do it and how to do it - formulations are different for cats and giving them something made for humans or other animals can poison them.

Don't know about vitamins, but when one of my foster kittens got ringworm (the others were negative) we put him on Lysine gel. He still gets it and he's been ringworm-free for months now.