My standard advice for seasonal allergy sufferers, regardless of species :)
Wash bedding regularly
Allergy meds regularly - do not skip a dose, ever or you will regret it. Once the allergies have hit, its much harder to reign them back in, and takes longer.
Take showers/baths at night. for a dog, you'll need to wipe him down with a clean towel and plain water when he comes in from outside.
If it's really bad allergy monsters outside [allergens], hide inside.
But realistically, since this is new, you should get him checked by a vet.