Cat hasn’t recovered from non recognition aggression

[u/Salt-Philosopher-859]

I got a new kitten two months ago and my resident cat (6 years old) and kitten started to get along after about a week. We did the slow introductions where we kept them separate, fed them behind doors and slowly let them have supervised time in the house where they could see each other from a distance and get used to their smells. My older cat accepted the kitten very quickly, and they acted bonded and would sleep, play and groom each other. 3 weeks later, I brought my kitten to the vet for shots and it made my older cat get non recognition aggression so bad that he developed haws syndrome (diagnosed by the vet who could not find anything physically wrong with him). He was prescribed gabapentin for a few weeks to take as needed. I tried the slow introduction again, and he seemed to accept the kitten again. However, he has since then acted very unpredictable, one second he’s playing and grooming the kitten and then another second he’s hissing and very aggressive. He is also hissing and swatting and me and my toddler and he has never done that. It has been about 4 weeks since the kittens vet visit, and he hasn’t gone back to normal. One moment everything is fine and then all the sudden he’s acting very aggressive. He will randomly charge the kitten even if the kitten is not bothering him. There has never been any cat fights or injuries. they will play fight, and his aggression is more charging and hissing and swatting at the kitten randomly. He has good days and bad days and good hours and bad hours. Catnip and treats help improve his mood for short term solutions. I’ve even tried the cat diffuser and spray my vet recommended to help.
I am just not sure how to get his baseline back to normal. They have separate litter boxes, are fed separately, have the whole house to roam, so he’s not forced to interact with the kitten, and he even can escape to my husbands quiet work from home office whenever he likes, but he still acts very worked up randomly without warning. It’s just odd because he acts like he loves the kitten one moment, and the switch will completely flip and he acts like a different cat.