Poor kitty, I know how I feel when MY allergies act up!

Since you have had the kitty 8 years, think of something you have just recently added to the house, along the lines suggested by Russian Blue. Did you get new carpets? Could be the rug, could be the adhesive. Did you decide to switch from Lysol to Oust, just because of a sale? Could be something like that.

Doesn't sound like a food reaction if he started almost as soon as he got in the house. What type of heating system do you have? Forced hot air? Maybe the vents need to be cleaned. I did a thread on that a bit ago.

http://petoftheday.com/talk/showthre...forced+hot+air

Maybe these ideas will give you some help in tracking down the source. Best wishes.