Dont know how to hide it?

It is wrong for him to say that. Thousands of people that self harm work. He definitely shouldnt discriminate you because you cut. I would tell him that is unreasonable, maybe get someone from your mental health team to contact him? Make sure you are definitely ready to go back to work, it is a huge step. If you are and he still expects you self harm free then just cut somewhere else until you feel ready to be able to stop. I understand that it isnt just as simple as stopping. So cut somewhere else and only show him your arms. You do need to work out how you can manage without it too though, which is what im doing atm after years of cutting. It is hard work and Im only managing because I felt I was ready to stop. Good luck x