In my opinion, "He sings well" implies that you find the singer to do a good job, and that you enjoy his music. "He can be a good singer" implies that you often enjoy his singing, but there are times when you don't like it. "He could be be a good singer" implies that either you don't have enough experience with his music, or that he still has a long way to go to become good. In either case, I would take that sentence to me that you do not enjoy his music at this time.

Thank you XD