How can I help to improve my son's tennis skills?

There are several options, depends on your son's current age, level. 
If he is below 10, and he just starts, the most important thing for him is keeping interests in tennis. So you can find some local tennis group classes for him, or even semi-private lessons with qualified coaches. 
If he is over 10, and he has some basic level skills already, he could attend some tournament (USTA junior circuit, L7 or even L6), or he could join one USTA jtt team, to play for the team with his friends. Also at this age and level, it is also good to have one private coach for him.
If he has already shown some competitive skills in this sport, you could also find some "big" tournaments, for example, Little Mo Tournament which is national wide tournament for young kids. You can even bring him to watch some live professional player's match.