Start with the items dllawson addressed above.
Beyond that, 12 accounts is not a company and the likely reason that he wants out midway is they are not priced properly for his skills, equipment or perhaps even for the market.
If you can turn a profit on them, pay a sales commission to him (that rate depends on your profit margins on these accounts, 10-20% would be reasonable if they are priced properly) for the remainder of the year. If they are not profitable, he can refer them to you as he quits, then you can price them accordingly. That only leaves him with some goodwill for his now former clients that he helped locate a replacement for him as he quit midseason.
I don't think I'd consider any other arrangement.