Why is his cough so hoarse?

My son will be 4 months on Christmas & he has a cough that sounds hard and like it hurts him to cough! it's so hard that he lost his voice from it. I hate to hear him cough because it sounds like he's in pain. I kind of think it's asthma cause me and his dad has it but mines never bothered me but his dad's bothered him bad when he was a kid. Any ideas on what it could be or anything to help. Thanks in advance. oh he's goes on his 4 month check up on the 27th