Maybe I'm more strict on the approach, but I would find out who was bullying him, and suspend them so they can undergo mental health evaluations and mandatory home interviews. What is going on at home? Do they lack attention from their parents? Are they not ever disciplined? Is there undiagnosed conduct disorder? Additionally, parents will need to join their children in some way, to ensure they are actually participating in some way. Failure to do any of this may mean that suspension is indefinite and schooling will be conducted at home or elsewhere, but with a heavy focus on them also doing their required tasks.
I'm sorry, being a shitty human being, adult or child, is no excuse and is a sign something is wrong. This might sound harsh but I would give none of them a quarter. Bullies often have undiagnosed disorders, or the possibility of. They can have a crappy homelife too, so forcing parents to have some accountability is crucial.
As for the child who is bullied, they too will have mental health evaluations, but maybe do fun activities with other children, such as extra field trips, pizza parties, movie nights, and maybe to take trips with other students.
Good behavior is rewarded. Shitty behavior must be disciplined appropriately.