First off, let me say that you are doing an excellent job with this kitty. You have earned his trust and love. That is the biggest hurdle. Don’t worry too much about him being upset after being trapped. Yes, he may be very upset. You may have setbacks. This is all normal when dealing with strays or ferals. It is something that can be handled after you do what is in his best interest. You are saving his life and health. I have brought in a huge number of rescues, feral and strays, and they have all forgiven me for caging, transporting and the Spay/neutering. Some take a little longer than others but most forgive quickly. Getting him checked out may seem tough but it really is the kindest thing you can do for him. Neutering is a must. He will live a better and longer life after this. Flea elimination can take a little time. If he has been living in your garage, you might want to clear it of fleas while he is at the vet. I use a flea spray and then vacuum and finally mop by soaking the floor with Dawn dish liquid. The cage is wonderful. I have used cages too. As long as the kitty has room for a perch, food, water and litter box as well as a little room to move around, he can be in there while you work with him and socialize. I just had a friend purchase that same cage to integrate one of his cats into his main living area with his other cats. Felv is not a death sentence. I have many cats. One is FIV and has heartworms, another is Felv. I have to keep the Felv cat separated from the other kitties but he has been a very happy and healthy fellow for 11 years. He was around many other cats in a colony before I got him. Mating, sharing food/water dishes, litter boxes and fighting can spread this illness. Living in the same house although in a seperate space, has not spread the illness to others. I brought in other cats from his colony during the same time frame who did not have Felv but none have ever contracted it by living in the same house. Felv cats can live very good, long lives. The disease may not be active. Kittens, very young cats and old cats have the toughest time with Felv. FIV is not a severe issue for your house cat. The biggest risk with FIV is to the cat that has it. If any other cat gets sick then the FIV kitty is at risk of catching what the other cat has. FIV kitties can live normal life spans. You asked how long you need to let kitty heal after neutering. Since you will be working to socialize kitty and have the cage, you shouldn’t need to worry. You will be working to socialize past his healing time so if you keep him in the cage until you see he is trusting you to work with him you will be able to observe that he has healed properly. Males heal faster than females. If all looks healthy at surgical site after a week, you should be totally in the clear. Ferals that go through TNR are often released back outside very soon after surgery but I prefer to keep them longer to make sure healing is complete. Retrapping is very difficult if any further vet checks are needed. Concern about spreading anything is sensible. Wash your hands well. I made it a habit to wear a particular set of loose clothes over whatever I was wearing for the day. I would keep these clothes at the door to slip into when I worked with new cats. Just as a vet wears a lab coat and washes up between patients, you should do the same. I probably took more precautions than needed but never spread anything short of a possible flea. Hopefully, not even that. I did shower immediately after a day outside with the colony to prevent fleas from hitchhiking. Lol You are well on your way to socializing already. Just continue working with him as you have been. If you need guidance on that later, many here can help you. Please continue to keep us updated. You are a kitty guardian and that makes you a kitty hero! Bravo! Thank you for saving your indoor kitty and this kitty!