My question is should I recommend he go back and amend this?

Yes - and as Bob points out, tell him what the consequences might be if he doesn't.

I will not prepare his tax of course unless he agrees to comply. 

You *can* still prepare his taxes going forward as long as the filing status is correct.  Personally, I wouldn't want to work with a client that refused to amend/correct the prior year returns (assuming the impact is significant....).  Life is too short to work with such folks, IMHO but YMMV.