You have some beautiful kitties! I suspect your little boy is feeling a bit intimidated by the larger cat, who is after all a complete stranger to him. You might try crating her while you let him out in the room with her for supervised visits - this way he can get to know her a bit better while keeping both of them safe. I would definitely continue to feed them next to each other with the gate separating them. You can also play with them side by side in the same way with wand toys or laser lights. You didn't mention how long you have had the kitten in quarantine, or how long you have been attempting to introduce them. Sometimes kitties hit it off right away, other times they can take weeks or even months before they are truly comfortable and happy with each other. It is important to be patient and let them work this through in their own way and timeline.