No, not in my opinion, I don't believe that he is behind. It seems as though he is doing quiet well. Addition and subtraction is not that emphasized or of that much importance at 5 years old and seems to be taught in 1st grade. You may want to begin practice and explaining how addition and subtraction works, but I would not become too distracted with the details of math. I think that at 5 the phonics, reading and writing is much more important.
A fun game I liked to play with my daughter with Math was by using craft sticks (available at the craft store or dollar store) they usually cost $1 for 250 sticks. I would lay several out and we would count the sticks, then separately, I would lay out several more and we would count the sticks and say for ex. 3 + 3 and count all the sticks equals 6. Practice subtraction using the same method. Education.com has several great worksheets available that you may enjoy. Sounds like your son is very smart and I can tell that you are a caring parent, so I believe that your son will do very well with his school work.