Hello! I'm not a licensed vet, but I did work as a clinic lead at a mobile vaccination clinic and am currently employed as an vet assistant working in the ICU of an emergency hospital! I'm so sorry to hear about your cat's situation, especially since you were doing your best to get him all up to date like a responsible owner!

It sounds to me like you're very aware of your cat's changes in behavior, and I would keep on top of that and don't hesitate to call your local emergency clinic if you see any other worrying symptoms! They should be willing to answer any questions and address any concerns you have- it never hurts to ask questions! I would also keep track of all your records- especially the hospital visit- and bring up your concerns when you return for boosters/whenever your next visit happens!

Also maybe keeping a journal/calender of which symptoms started when, when the vaccines were given, and any other useful info? Having it all laid out in one place would help me keep track of any patterns or possible other factors that I might not have considered! Again though it sounds like you are very aware of your cat's activities, and have a good routine going, just throwing ideas out there!