There are two schools of thought on this. If your son is playing in secondary or high school level and is advanced in maturation such that he plays other sports, ie., football, basketball, etc, then his arm can get some rest while doing those other sports...except if he plays QB, of course.
However if you really want your son to achieve the maximum potential of his natural abilities in baseball, then the one and only sport he should be playing is baseball.
Some kids, like my son, wouldn't think of taking time off from the game, becuase they know, from experience that there is no neutral position for development...either they are progressing or regressing. Regression of the both physical memory and mental memory skills starts within hours of being idle.
My son, plays year round and loves it. Its more of a grind on us then anything else.