My Husband Has Had A Cough, Just A Cough -

My husband has had a cough, just a cough - he hasn't been sick, not too bad but a constant congested sounding cough for a few weeks. He says he feels fine. He's a little tired but he also gets up at 4:30 a.m. for work and works late at night on Saturdays. Is job is also physical. He is soon to be 64. Any idea what might be causing the lingering cough?