OP, just wondering if your DS might be on the quiet side? Sometimes teachers, perhaps understandably with little else to go on, assume being loud and confident translates into 'will be good on stage'. They might be concerned that a quieter child will seize up in front of an audience or won't be able to rustle up the required razzamatazz.
In Y5, DS, a low-profile sort of child, was given the least number of lines in the school play out of the whole class. Knowing the children's personalities, you could see that the size of part a child got was strongly correlated with how extrovert that child was.
The day after the actual performance, a teacher came up to me in the playground and said, 'Everyone in the staffroom was surprised how good DS was - he's usually so quiet!'
When Y6 rolled round, the teachers remembered DS was capable of on-stage pizzazz despite off-stage quietness - and he was given the main part in the play that year.
The moral is, I think, to make the most of the smallest opportunity. If there is only to be one line for your DS this year, let that line be delivered with clarity and conviction. Add a sparkle in the eye and a sprinkling of stardust .... and who knows what next year might bring!