Unfortunately, there is nothing you can do from the outside but complain to the institution. This could have a double-edged result though. Letting someone in the administration know of these details will certainly shake things up there for a while. They will do shake-downs trying to find the shank. But these things never stay quiet and he could end up having more problems from you saying something than if he just lets it go.
It sounds like your son has a group that helped him through this incident and it might serve him well to stay close to those guys and just avoid the people he had a conflict with. Usually these incidents are done to test an inmate. Since there was a push-back, the people who harassed him might look for another victim who won't push back. These things usually die down quickly.
https://www.inmateaid.com/ask-the-inmate/my-son-is-afraid-for-his-life-he-was-robbed-last-week-by-some-gang-members-one-of-them-had-a-shank-some-of-the-other-inmates-stopped-what-was-happening-and-he-got-most-of-his-property-back-but-now-he-is-afraid-they-will-get-someone-to-get-him-back-my-son-#answer
Accepted Answer Date Created: July 28,2015