3/10. Here's why:

I don't see any inconsistency in the character's personality. I think it's common for children or teenager or even adults to behave rather differently in different situations. Nigel's character was a bit complex. At times it seems that he wanted to please his parents and try and be a good boy, while at other times he can't resist to act like a brat. Transition from a child to teenager, more often than not, does change a person's character.

Teenage Nigel's portrayal by Highmore wasn't bad at all. He seems to have developed a habit of repressing his emotions while trying to stay brave or confident at the same time. Which is probably because his father and late mother, or anyone else, never really showed that they understood him, or even showed signs that they tried. Whenever he expressed his emotions or feelings, the outcome had always been unpleasant. Being a homosexual might have added to his out-of-place behaviour, especially since it was in the early 70s. I imagine he must have felt lonely and misunderstood most of the time, and that lead to his rather stoic exterior. But through Highmore's performance, you can tell that Nigel had various sorts of emotions even though he always tries to repress them.

I think Highmore actually did a good job in this film. Just my opinion.