Could be! Enforcement of this kind of thing varies a lot from jurisdiction to jurisdiction, and from case to case. I seriously doubt that he would have to show proof that he is going to church every single week. But the fact remains: if the judge found out he was NOT going, and decided to make his life miserable, he could revoke his probation.

The answer to your second question is yes, but the problem is that you have to have someone who is willing to challenge it. The alternative for this kid was prison - so if he challenged the judge's sentence, he might win, but he'd end up right back in the same court to get sentenced again, and the judge may not be so lenient the next time!