You really need to get talking with Mrs Nick. I believe that a joint approach is the by far the best way. It would show him that whatever the differences are between the two of you, you both have his welfare at heart. And I feel you should really impress that to Mrs Nick. She must feel the same way.
I take it you don't have "THE BOX" but I would suggest getting hold of it because, if as you fear Mrs Nick won't agree to do this together then, you'll need it for your long walk and talk.
Finally though, it will be the Boy's decision as to what he does and I would advise backing him fully, whatever you personally feel about it.
Best of luck Nick.