How do I keep my client from pulling off his ostomy bag? Do you know of a product to cover them?

Agree with anti-strip clothing (aka "adaptive" clothing for people with dementia).

We had great luck with soft abdominal binders with velcro to the back, but you do want to be certain how to apply as you do not want to have it too tight, while fastened well enough to keep it on.
I sure wish you good luck, and as an RN, can tell you that your worries WILL come to fruition on this one.

I would try anti strip clothing - there are several suppliers (like Silverts) or they are available through Amazon