I don't think you can get his name on the birth certificate either. And, tbh, I don't think you should do that because that is really lying, as opposed to just not telling him the truth (fine semantic point there).
I'm a bit torn here - someone in my family has an "adopted" DD and I pushed very strongly for her to be told (she was when she was 7) because I felt it was the right thing to do - but they know who her bio-mum is and the whole back story so it's very different.
In your case I think the problem still stands that your DS may come to find out in later years and it could devastate him to find out that he's not who he thinks he is. So I do think that it's probably a good idea to mention that his father's sperm didn't actually create him, although he's still his dad in every important sense of the word - as to how you deal with the fact of his conception, well in some ways it can be useful for youngsters to know their parents are/were fallible, but that they managed to turn their lives around and you'd never know now!
But you need to get on with it if you're going to do it (and I think you should, really - it's horrifying how these things have a habit of coming out unexpectedly, usually through some kind of family testing for donor matches or health conditions or some such) - the longer you leave it, the worse the reaction is likely to be. My family member wanted to leave it until the DD was 18 - just about the worst possible time to do it, IMO - but was persuaded otherwise.
Someone has already mentioned www.norcap.org.uk/ - they are an organisation that should have counsellors who could help you work out the best way to approach it.