I have similar problems, and while I haven't bought an air purifier yet, I have the Molekule Air on my wishlist. Definitely going to research these others as well now!
A diffuser and some peppermint essential oil will work 10x better. When I travel I mix a little tea tree oil with some of that peppermint essential oil in a travel size bottle and rub it under my nose and it works miracles!
Any purification system from Aerus is great. The AP500 Beyond by Aerus is reasonable and covers up to 500 sq ft. It destroys over 99% of all surface and airborne contaminants. Very low maintenance and easy to use. Highly recommend!
VAVA Air Purifier for Home Large Room with UV Light Sanitizer, Eliminates Germs, with H13 True HEPA Filter, Remove 99.97% Dust Smoke Mold Pollen, Available for California
This is probably what he really needs. After 20 yrs living in FL I moved back here a few years ago and my allergies were horrible. I sounded like a congested frog. This is what cleared me up and now my 6yo son takes it too. It’s the Costco brand of Claritin and it on sale fir $7.99 for a year supply.