Can you just tell him that he is really important to you, and it helps you a lot to have someone who cares about you like he does, but this is just a subject that is hard for you to talk about and you aren't ready?

If you are ready to talk about it, or you want to be ready, there are some good books about SI, as well as websites. I posted a couple of websites in the thread at the top of this forum - they have sections for explaining SI to loved ones. A good book about it is "Bodily Harm" by Karen Contrerio and Wendy Lader, but there are others too. This one is good for explaining it to loved ones. The books and websites can give you help and ideas about talking about it, or you can also ask him to read the information there and you can talk to him about it afterwards.

Have you considered going to therapy? I'm glad that you have a friend who is supportive, but being your only lifeline is a pretty big burden. I bet that he would be more comfortable if he knew you were getting professional help, and besides, you deserve to be able to recover from this, and getting professional treatment really is the best way to overcome it.