Does your friend have any help like a therapist or anything? I know that "get help" is hardly what you want to say to a person in that situation.
As a cutter, I don't think there's ever really anything that a person can do to help me "in the moment." It is all about being open about it when he is not in a moment of distress. And chances are he wants to stop too. So practical suggestions help...it's always nice to have a friend to problem solve with.
Ideas that you could give him (or even do together for support and accountability...):
1) Get an empty milk jug and fill it halfway with water and put it in the freezer. When it's frozen dump all the "cutting tools", razors, etc, into it, fill it with water, and place it back in the freezer. If he wants to cut, he has to wait for the ice to melt, and that buys him time to rethink things. It's often hard to throw razors and stuff away, so this is a good middle ground.
2) A less "extreme" form of this is placing "doors" between him and his sharps. Put them in a box in the garage, or even in a closet. Every door is seconds added to the amount of time he has to consider his actions.
3) As far as the pain goes, holding chunks of ice in your hands (or on more sensitive areas if needed) often provides the same relief and can be quite painful without the longterm consequences. I do have a hard time with justifying this for myself, because it's still giving in to the need for pain... but it can be useful as an interim solution.
4) It doesn't work for me but some find that drawing on themselves with a red marker can provide relief... some even get quite creative and that distracts them...
Ultimately your friend needs help and support. But I hope these suggestions help him in the meantime.
I know that it can be hard to see your friend hurting and in pain but I think the best thing that you can do for him is to be a true friend and offer to support him but encourage him to get some professional help especially for the cutting. He is definitely in need of more help than you are able to provide for him and although you can remind him that you are not going to turn your back on him like the other friends, he does need to talk to a school counsellor, therapist or doctor. Things can only get worse from here if he doesn't get help.
I am not sure in which country you live in but I am sure if you do a search in the phone book or the internet for your local area you will find some resources that will be helpful.