I have this idea, be honest and but firm, and do a proposition to him: you'll allow him to stay and continue working but he has to accept taking a serious psych

I have this idea, be honest and but firm, and do a proposition to him: you'll allow him to stay and continue working but he has to accept taking a serious psychological treatment, that person needs mental attention immediately. He sounds like a reasonable/intelligent person, try to negotiate something that helps him and you.