Well, first of all, what your friend needs is love.
He needs you to be there for him. After all, he was there for you.
Cutting is a complicated situation... I, myself am a cutter, so I understand the reasoning behind it.
Ultimately, your friend needs you to love him the same way you did before...maybe even more.
Do not judge your friend, that will only make it worse!
Oh, by the way, simply because your friend cuts DOES NOT mean he is suicidal!!
I hope this helps...