Sounds like he needs to be in therapy to me. Holding on to that anger from childhood is unhealthy. I don't get why someone with a degree in computer science can't get a job in that field. I think he needs to find at least a job at some place while looking. Your offer to support him while he does is nice. It doesn't even sound like he appreciates that. He needs advice from someone. When children mature they often see that their parent can be a good source of advice--because they have gone through a lot and want the best for them.

Just my thoughts. You are more than meeting him halfway. His choice: Do what you say or it's back to his sister's.

P.S. I am mentally ill (bipolar) and have two sons in their twenties. They both live out of state where they have found good jobs, despite my failures as a mother because of my illness. We helped them through school and grad school. One lived with us while finishing his Master's in the same city, but he paid a very modest amount of rent. They've both been in therapy and I encouraged the older one to go back into therapy because of his anxiety. Sounds like your fellow is cutting off his nose to spite his face.