Naam, he should contribute, she is his dd.
Siobhan, difficult situation...erm... requires some thought... My gut instinct is the same as yours (fear that she may become pregnant) but you are right you are not going to stop them from doing 'it', all you can do is to make sure dss has and uses contraception! Although I don't know how you would enforce this either, you can't stand outside the bedroom door shouting "And make sure you use a condom!"
Has your dp spoken to dss about contraception? You never know, if the girl is as sensible as you say she is she may have even got herself on the pill. It is up to her to tell her mother they are having sex, she is 17, its up to her really. Its not your responsibility, so don't feel guilty. Do you get on well with the girlfriend? Maybe you could have a 'chat' with her about your worries that she may fall pregnant?
I know dp gave ss condoms when he was 18 and it became clear to us that the relationship he was in (still is 2 yrs later) was becoming serious, we spoke about it and I knew that ss would have been embarrassed at me giving him them, so dp offered to give him them....Oh no, actually I remember.... He put a pack in his Cornflake packet then asked him if he got the 'free gift' in his cornflakes, whilst giving him the 'knowing nod'. It was hilarious and broke the embarrassment surrounding the situation.
I’m not suggesting that you do that BTW, its just different ways for different people IMO. But I do feel that communication is the key.