Maybe suggest that he sit with the children he gave his lunch money to in the past or anyone else he has helped. Just go up to their table and sit down like he belongs there. It is better if he can ask if he can join them today, but some kids need to learn self confidence before they can open themselves up to rejection by asking first.

Befriending his bully is actually a very good thing. Bullies need friends too and one of the reasons some bullies pick on others is to gain attention because they think no one cares about them. If your son can help his friend to feel liked enough that the bully doesn't "need" to hurt others as much, then he is doing the whole class a major favor as well as helping the bully find other ways to solve his problems.