Bertolucci's direction
The last time I remember seeing this movie, my impression was that Bertolucci, at the time he made it, did not have enough familiarity with spoken English to judge the deliveries of his actors. This would be especially problematic with the boy's performance (his first movie). The director needs to be extremely keen to inflection, tone, emphasis etc, being the one who is making it all fit together and making it all add up.
But the film, and Matthew Barry's performance succeed in spite of that.