Rather than taking his PC away, you might consider moving it into a shared area of the house and setting a curfew on it. My own experience with depression has led me to believe that some of it is caused by light-exposure and stimulation before sleep and that having a device-free wind-down period can help.

If the final 2 hrs before sleep were a time for non-screen activities (reading, board games or just general family talk), that might be a good compromise between letting him keep it and taking it away entirely.