Wow, how horrible
Unfortunately I went through the same thing, just a little older. First, do NOT worry about your son hanging out with him!! This is typically a way to deal with stress, because you are not able to vocalize it properly. Most of the time, you don't even know someone is a cutter, because they seem ok on the outside. They are self harmers, but will not harm others.
I personally would not talk to his mom, unless you have a good relationship with her. She obviously knows about it, and may be angry and/or embarassed. I think talking to her would only upset her more. But, if you have a good relationship with Reiss, I would let him know that you are willing to help him if he needs it. Let your son know that too, if his friend needs help, then he can come to you. He may be afraid to talk to his parents about it, but may be more comfortable with an adult that he trusts.
Cutting is handled by a therapist/psychiatrist, maybe they have one at the boy's school, or a local wellness center? It's a really tough situation, because he needs to want to get help, but you can't force him (especially at that age). One other thing I want to throw out there. The cut was obviously bad enough for him to go to the hospital. I'm sure there are doctors/nurses on the board that can verify this, but the same thing happened to me and I was put on suicide watch, and sent to a mental wellness hospital for three days. It was horrible, but it was exactly what I needed, and I got all the help I could have ever asked for. Hopefully some of the hospital staff recognized his cry for help and have recommended a doctor for him to see.
Sorry this was so long, this one really hit home for me
I hope everything works out ok. Hang in there!