Hi C.,
I looked up room air purifiers on Consumer Reports online for you and it says that the Whirlpool Whispure AP45030 S is the best model for the money (it costs $230). I have attached part of the article for you here. My daughter also suffers from seasonal hayfever. It might help to also get new carpet or hardwood floors to help control the dust in his room. Good luck with everything; I wish you the best!
Best room air purifiers:
Whirlpool Whispure AP45030S $230
Kenmore Progressive 83202 $270
Both combine fine performance with quieter operation, especially when set on low. The Hunter Permalife 30547 costs a bit less in annual costs but was noisier when run on high. Though the Friedrich C-90B and Kenmore (Sears) K6 85264 were better at dust removal on their high settings, they emitted moderate amounts of ozone in our tests, so we don't suggest you buy them.
No air purifier alone will clean the air. Nor should it be the first thing you try. Based on increasing concern about exposure to even small amounts of ozone, we strongly suggest using whole-house and portable air purifiers that rely on filters and produce no ozone.
At their lowest and quietest settings, the top-rated portable air purifiers outperformed or matched most of the other portables in the Ratings. There's no reason to choose any others.
Stick with whole-house systems if you have forced-air heating. The top-rated furnace filters are a simple, inexpensive upgrade from the standard furnace filter. For performance comparable to that of the best portables, opt for a professionally installed system.
Ventilation is still the best way to remove odors from your home. Although our latest tests show that the IQAir HealthPro Plus and Austin Air HealthMate HM-400 removed cooking odors, that capability will be reduced with use. And those models aren't very effective at removing dust or smoke.