Please be patient. He is only five years old. He will become more focused with time. Rejoice in having such a clever son and give him time to grow up.
Smart children at this age have very active minds. They are at the point where the whole world is suddenly open to them. The smarter they are, the more they try to take in. Its not that his attention span is short, it just that he moves on to other things a lot faster.
There is a series of books called the learning to get along series that might prove helpful. They are designed for the 4-7 year old child and are meant to be read aloud. Several might prove helpful. But you might want to check out "try and stick with it". check out this link
for more info. http://www.amazon.com/Try-Stick-Learning-Get-Along/dp/1575421593/ref=pd_sim_b_8
Do read to him every night. Probably the most important thing is laying with him and talking. He will worry, you can calm those worries. Sounds like a neat kid.