Just a note to SimLondon and the OP from a former lawyer. If you are not married you have no right to his house if you split up even if you have been paying the mortgage and contributing to household expenses.
That does not change even if you have been together for twenty years. Nor will you have any rights to maintenance from him if you split, however long you have been together. Unfair and in my view wrong, but that is the law as it currently stands. If you aren't married, you don't have any rights if the two of you split.
What you want is a formal document which acknowledges you own a real interest in the house (what you refer to as "going on the deeds"). That will either be a legal transfer by him of part of the property to you or, as other posters have said, a declaration of trust by him in your favour i.e. a legal document where he says "It may only be me whose name appears on the title of this property, but actually it is really owned in x proportions by me and OP and I hold it on behalf of both of us".
These are both legal documents which need to be done correctly so see a solicitor or licensed conveyancer. Depending on the value of the house there may be stamp duty payable.
The mortgage, as another poster said, is a liability - a debt owed to the bank. The bank will be very happy if you agree to be jointly liable on the mortgage with him. Effectively you'll be his guarantor. Don't do it.
The only circumstance in which you should even consider becoming liable on the mortgage is if he transfers a decent chunk, ideally 50%, of the property to you (because you become liable to repay ALL the mortgage if he defaults and the property can't be sold for enough to repay the mortgage). I bet he won't do that.
If he transfers any part of the property to you the bank will probably insist that you do become liable under the mortgage so make sure that the value of what you get is worth the liability you assume. Take advice from a solicitor before you do it. The bank should insist on this anyway.
It sounds to me as if he is well aware of the position and likes keeping everything for himself, thank you.
If he won't transfer the property to you and you want to stay with him then I guess the way to look at it is that you would have to pay rent to live somewhere. Work out what the going rate locally would be to rent somewhere for yourself and any DC and don't contribute more than that plus a reasonable share of food and bills. That way you are not losing out financially.
Sorry, I meant that to be a short post!