Like others have said.. you are just more fun. Sounds like your dog's relationship with your son is like Titan's with other dogs.. he wants nothing to with them after a minute figuring out that mom is the one with thumbs and can throw things. Other's say Titan is "rude" to their dog, and that it just wants to play, but Titan doesn't play with other dogs. He plays fetch with mom because that's much more fun!
The relationship will grow when your boy is older probably and can start doing things with the dog alone.. which as other pointed out is not now. lol.
Commander Charly Titan (GSD)-15 Jan 2010