Depending on the age of your son, why don't you ask him where he really wants to live? and let him learn on his own that the easy way out isn't always the best way. I know it would be hard, but maybe by making some good points to your son, he may change his mind. Ask him where he wants to go in the future, and if he wants to go to school for a specific profession and what not. Ask him what exactly he wants from them that he can't get from you or Dad. Be open and honest, and you just may be surprised that he's not as bad or lazy as everyone thinks. If he insists on going with the relatives, then you need to sit down with them and create a parenting plan with them, let them know that you are not just going to "roll over" and do some compromising. I know it's hard to let your child go, but in the end, he will grow up, and if you let him make his decisions, he has no one to blame but himself. I might be a little brah on this but i think, depending on the age, you can't really control what that child ants, they will find one way or another to get what they want. Lie, runaway, whatever it takes to get there. But maybe, just maybe, if you are honest and open to him, he might see you as a person, instead of a parent.
Good luck,
Skorpio