I wouldn't. And I'd be asking for rent too...
If you got married then maybe you could look at doing something but even then I'd be doing it as tenants in common and that he would have a significantly smaller share than 50% - maybe just 10% or something of that ilk, plus I'd protect it with a pre-nup and a post-nup to ensure that I didn't lose out big time. Does he have anything to put into the house for his share - other than the bills which don't count (otherwise all those people that rented properties would be asking to go on the deeds of the properties they rented - bet he has rented previously and never asked to go on the deeds despite paying his share of the bills...).
If you are happy about him not paying rent and effectively subsidising him then why not suggest he uses his share of what would otherwise be rent to put into buying a buy to let property so that he doesn't lose out on the property market if that is what he is worried about. I don't know if you would want to go into it with him - and then on the offchance you split up, you would have your property and he would have his / or a joint one to split between you.
Just out of interest - how much of a share does he think he should have if he were to go on the deeds? And what does he think he should pay for his share, other than just paying his share of the bills? Have you asked him directly if he is expecting you just to gift him £xxx,000 just because you are in a relationship together?
I think you should listen to your inner voice that is wary of signing anything over and just laugh, as you keep telling him that you'll think about signing over half the house to him when he has enough money to buy half the house from you...
And if you do get married - I'd be getting very good advice on how to try to keep your house from going into the shared pot, because at the moment it sounds more like he is using you as a quick way to get half a house and stop having to pay any rent, rather than being in a relationship where he really loves you as if he did it wouldn't make any difference.
Sorry, I know that's extrapolating wildly from your two posts on the thread but that was my immediate reaction to what you'd written and it seems that you sort of see it that way too...