[RottsNScotts]

Sounds like you need to do a little bit of "training". With someone on the line to the collar and behind the dog, have your son show the dog the toy and encourage the dog to follow your son as he backs up (can't sit if he is moving!). When the dog is pulling a bit, tell him "Get it", loosen the leash enough that he can reach the toy where it lands and have your son toss the toy behind him so the dog runs by your son to get the toy. Play, play, play! When the dog has figured this out and is immediately pulling towards your son, briefly - and I mean really briefly! - hold the leash and do not let him go forward. Make the pause SO brief that it doesn't even occur to him to sit before you tell him "Get it", release him and toss the toy.

Slowly increase the time he has to "pause" before being released with "Get it".

If he really is not interested in the toy, you can do the same with a treat; just make sure it is big enough that he can see it and find it immediately once it is tossed.

If there is still trouble, the toy may being held too high which encourages the dog to raise his head which often leads to lowered butts. Try lowering the toy to start, even wiggling it on the ground.