Thank you all for your thoughtful responses. Of course, I know you are all right and I followed your advice. I appreciate your support:)
I agree despite his mental condition he is still an adult. If you try to force him to get it removed it may make him feel as though you are treating him like a child. Which could possibly cause him to be rebellious towards you, and cause conflict with him. I think the better approach may be to suggest it and let him know if he ever wants to get it removed you will pay for it. I think in this situation it is more important to be understanding rather than demanding. Understanding someone with a mental illness can be one of the best medicines and comforts for them. Demands are more like a poison for people with mental illnesses for the stress that it can cause may be more than they can handle.
I would not 'make' him do anything. He got the tattoo for a reason, and until that reason ends he 'needs' it there. Forcing him to remove it may cause further destruction like moving into cutting to release those emotions. There are things worse than tattoes and it is on his leg not his face or neck. Most people are not going to know the significance of 5150. I didn't. I wouldn't force him, but I would encourage him to get tested for hepatitis depending on how clean a shop he got the tattoo from.
I would state that you love him and don't see him as a 5150, and if he ever wants the tattoo off you'll pay for it. That lets him know he is loved. The other - it is power and control and it will backfire. He's 20, gotta let him make his own bad decisions.
Wish it were easier for you. I am sorry he was in a bad psyc ward. There are good ones. Unfortunately they don't come with a rating. You did the right thing though. You couldn't have known how it turned out.
No matter that he has mental illness issues, he is still an adult. He made a choice an adult can make. I will share how you feel about the tattoo and offer to get it removed should he ever wish to have it done.