Anxiety about school swimming lessons - all advice welcome

My 9 year old son is stressing hugely about going swimming with his school class this month.

He did it last year for two weeks - with lessons every afternoon - but found it very overwhelming and the thought of doing it again is leading to daily meltdowns.

He is worried about his head going under - it happened last time briefly once - and struggles to get himself changed and re-dressed in time as he has dyspraxia as well as autism.

The smell of chlorine and the noise of the other children in the pool also cause him a lot of anxiety.

We have tried to reassure him things will be ok and also emphasised how important it is to learn to swim but fear that forcing him to do this may prove counter-productive.

He already finds school extremely stressful and seems burnt out by this. Are we pushing too hard or would allowing him to miss lessons with a view to me teaching him over time be a mistake in the long run?