HMO tenant mental health

Other

Tenant has lived in my HMO for 12 months, good tenant no issues or complaints until the last month...he seems to be experiencing a decline in his mental health. Tenants are complaining he is laying on the sofa on communal lounge all the time, often in the dark  sometimes talking to himself. He has been particularly aggressive towards another tenant. Angry with him about making noise and not putting the bins out. On one occasion he got very angry, threatened to kill him and made a racist remark. The other tenant is scared of him. I advised him to call the police of that happened again or if he felt threatened.

Another good tenant is also planning to leave as she feels uncomfortable around him.

What do I do in this situation? I don't want to loose good tenants because of one tenant. Also, I would feel uncomfortable putting in new tenants in a situation like this...

He has breached his tenancy agreement with the above behaviour. In response, I went and spoke to him and reminded him that threatening behaviour will not be tolerated and is in breach of his tenancy agreement and if I am alerted to anything like this again I will have to take action.  He understood, was calm with me but it is evident he is not well, I tried to get contact details of his mum etc (he is a young man in his early 20's) but he refused. He mentioned a psychiatrist, he is known to mental health services. He is still working and paying rent. 
Has anyone been in this position before? Not sure what to do...