I have no idea how to stop my boyfriend’s random sneezes

My boyfriend (20M) has been sneezing constantly since last weekend. Like every other minute, no joke. He’s been tested negative for Covid a few times, has no allergies, and there are no irritants in the air. He’s also generally healthy, aside from his asthma. For the past few days I’ve been so confused, trying to figure out what’s causing it, and I’ve done a few things but nothing has worked. I thought maybe it was the pollen or allergies so I bought him Claritin and it didn’t work. I vacuumed the entire house, thinking that there might be dust in the air, but nothing. He even went to the doctor and they said it wasn’t allergies, and nothing they could do could help. I’m seriously getting desperate at this point, and I’m at my last straw. I don’t know how much longer I can deal with him constantly sneezing.