All right then.
At the moment, you are the sole owner of the house. If you live together, you do not have to change that position, and it might be better for you if you did nothing and left yourself as sole owner.
As long as you are not married, his rights, if any, will depend on the general law of property and/or landlord and tenant. If you do not get married, there can be no divorce, and the Court would not have any powers to redistribute the marital assets as it would have with a divorce.
You mention him '' going on the mortgage ''. If he lives in the house it is is reasonable that he should pay something towards his accommodation costs. In time it is possible that, if he makes sufficient in the way of monetary contributions he might be entitled to an interest in the house, but not as high as 50/50 unless the two of you start to be known as Darby and Joan. You can spell out in advance what the share is to be, if you want.
He can''t '' go on the mortgage '' unless the property is in joint ownership. He could act as guarantor.
The important point is that he doesn''t have to be a joint owner or a guarantor to live in the house. He could live there for no better reason than that you allow him to do it; and from his point of view, his rights to occupy initially would be very tenuous.
LMM