If you're buying it just because you need another rifle, go ahead and get rid of the .204. The .223 is cheaper for sure. Also, I'm sure your son would appreciate his own rifle, so that might be a good idea, even if it will be a few years before he is ready to shoot it. Personally, I would get somethign a little different, but I don't have a son yet to shoot with me, so maybe your idea is best.
-Jake