hi and welcome. I am wondering if he's had a Vet checkup? if theres no discharge or malaise than it really doesn't sound like anything respiratory going on such as a URI , it could well be something is lodged in his nasal passage that he is trying to expel.
Cat allergies usually show up on their skin, such as itching, sores or scabs.
you say you've changed your cleaning products to something more natural, I hope this is vinegar, as most of the so called 'natural' products still contain alot of chemicals.
do you use scents in your house such as candles or room freshners or hairsprays etc?
any new furniture or rugs? do you use unscented litter? even dry kibble food,
all have chemicals. I would start with a good Vet check and an elimination process.
try to feed only wet food if possible...keep us updated, good luck