I truly think it has to do with his age at this point. Older cats often suffer the same symptoms and side effects after receiving vaccinations. It just has to do with a weaker immune system that accompanies older aged pets. If he does not go outside, I would stop the vaccines. I have an older cat myself, 10 years old, and I stopped her vaccines last year. She doesn't go out and the risks are not worth it.
I still however vaccinate my 3 year old cat.
In 2006 he was 9, and is was still considered a senior cat. Also it is so very common when you take a healthy cat into the vet for routine check ups and procedures, that they pick up viruses and bugs from other cats in the vet's office. Much like when we go to the Dr. or hospital. There are germs everywhere on the floor, tables, etc.
Just 2 weeks ago I took my son in for a sports physical, and I got strep throat 5 days later. I have no doubt I got it from sitting in the waiting room of my Dr.'s office.
So there are a few different view points you can take on this situation, but I do think in his age was a big part of the reaction. Vaccines stimulate the immune system, and these things happen as a result. Younger pets typically do fine with it, and older pets can have more complications with it.