Suspended from work for a crime that I committed before I was employed?

I was convicted for theft over three years ago. I started a new job about two years ago and there was nothing on the application form about a criminal record so I never said anything to anybody. Just recently my boss has found out about my criminal record and I explained it was for theft. I have now been suspended on full pay untill he decides what to do. I just wanna know if he is allowed to do this has my crime was committed before he employed me. I work at a petrol station and wrk by myself alot of the time and I am a key holder.