All the things other people have suggested along the lines of different pollens, lack of mould spores, cleanliness of/lack of soft furnishings, maybe different/newer pillows, temperature, humidity, and maybe things like different exercise patterns or less stress — plus one more left-field suggestion:
Is he having more caffeine than usual?
I ask because, a few years ago, I had to give up caffeine for medical reasons (well, I gave up all but my first coffee of the day, which you'll have to pry from my cold dead jittery hands, but as long as I keep it consistent it's medically fine).
A while afterwards, I realised I was getting wheezy every evening, and had to go back on the steroid preventer inhalers I hadn't needed for years.
Initially, I didn't make the connection, but then I discovered that caffeine acts as a bronchodilator, and realised that all these years, I'd been unknowingly medicating my asthma with coffee. It's described as a "mild" bronchodilator, and my GP didn't even know it was a thing, but for me, taken regularly throughout the day, it was startlingly effective. When I drank caffeine all day, I had almost no asthma symptoms. Maybe once every few months, I would react badly to an environmental pollutant and need to take my salbutamol reliever inhaler, but other than that, no symptoms at all.