This used to happen to DD in Y3 (developmental delay & difficulties with processing speed). They either made her miss her lunchtime to finish the work or sent it home to be done as homework. I made copies of the classwork she brought home and took it to the SENCo as evidence that she wasn't being given appropriately differentiated work.
I have had this happen to my dyslexic DS3, and we pointed out they (the school) were punishing him for having a disability which affected his ability to do the work at the same rate as his classmates. They were pretty awful about it (saying he's just lazy/not paying attention/in a world of his own) Kids need their breaks, and we successful argued that he should not be missing out on his breaks as it wasn't likely to be good for his moral or his work rate later in the day.
School have a duty under the equality act 2010 to make reasonable adjustments for pupils with SEN. The teacher should be differentiating his work so that he has adequate time to complete it in the lesson.