Hi k8,
As the others have said, I really cant think of any other way you can try to stop him, but the same thing happened to my SIL.
She had 2 cats, one of which was ours to start with but we rehomed with her when she was very young as her mum kept having a go at her. She had baby Samuel March '08, and she started spraying alot, even though she had been done. Even in the cot, the same as your boy has done! That was the final straw for my SIL, and she wanted her rehomed. Luckily she went to live with the parents of a lady I work with (the lady I work with had also taken on 2 kittens that my cats had!). She now has a fantastic life, I get regular updates and also pictures too!
You are doing completely the right thing, and atleast if he goes to family you can always find out how he is doing!
:-)