It could be because he has a block in his nasal passage due to deviated septum Or else it could be a condition called obstructive sleep apnea. If it is OSA then he needs to have a sleep study carried out. And then use an instrument which can keep his airway open so that sufficient oxygen can enter his body. It will also help him feel more fresh if sufficient oxygen is consumed while sleeping. It's called a CPAP machine. You can go to a sleep specialist in your area and enquire about the same.