Agree. Sinus infection, contact your vet. They may require a visit & or prescribe antibiotics. In the time being consiter any changes in the environment. Such as using central air to heat/ cool. Candles. (Smoking) anything that could be potential irritant. Hope for a quick recovery
Has he passed a stool since you noticed the change in him? If not, there very well could be a blockage. Be careful with the amount of pedalite so it won't increase the salt output in urine (creating crystals) dilute it with RO water if possible. Just trouble-shooting. Most definitely recommend...