Hi there,
My experience has been that it's not uncommon for exceptionally bright and talented children to impose high standards on themselves, and to develop perfectionist tendencies. Sometimes parentswhether deliberately or unwittinglycreate pressure in a child to try to be the best, but other times that pressure comes from within the child him/herself. Either way, it can be very stressful and lead to exactly what you say you believe you've observed i.e. a fear of having a go in case the child doesn't meet his/her (or others') high standards, and is thus exposed as some sort of 'fraud' who isn't as talented as he/she is 'supposed' to be. Once those fears and attitudes have set in they can be very difficult to change.
Your DS is clearly very talented musically, and I can't really see how you're going to be able to encourage him to work hard at the cello when he simply doesn't need to. He knows he can obtain distinctions with just a month of concentrated last-minute effort, because the standard required to do well in the exams he's taking is much lower than the standard he's capable of achieving. Therefore, unless he suddenly begins to take pleasure from the act of practising itself his attitude probably isn't going to change.
Things could actually be worse, though--at least he has been willing to put in that last minute work thus far. The worst of all worlds (IMO) is when a child gets so fed up that he/she decides to completely disengage, refuses homework and begins to fall behind. It can be very difficult for a child to find a way past the problems that kind of behaviour creates in time to live a fulfilling life later on.
Does your DS actually have to continue with the cello exams? I can see that he's going to need to continue to take lessons if he wants to continue to play in the orchestra, but it doesn't necessarily follow that he has to take the exams. Perhaps if the spectre of having to prove himself (even if just to himself) were to be removed then he would settle down and simply enjoy the instrument.
I wish you the very best of luck in dealing with this situation, because I don't think it's at all easy to find a solution. At the same time, though, it would be such a great shame if your son's choices at this early stage in his life were to deprive him of the joy of making music as an adult. Good luck to you and to him in finding a way forward.