Being on the deed versus being on the mortgage

So I added my boyfriend of 4 years to the deed of the house 2 years ago in which the mortgage is solely in my name- leaving him as an owner of the house but also not financially responsible legally for it - we have an extremely rocky relationship and sometimes talk about parting ways- the only issue is that he’s saying I have to pay him half of what’s been paid to the principal of the house in order to “buy him out” and It’s going on year 3 of owning the house- is this legal? He says if I get a lawyer they’ll tell me the same thing- I’m in Ohio- any advice?