My 6 year old grandson has a dry cough no other symptoms. going on holiday tomorrow.is it ok o go swimming?

Could you stop him?: With no symptoms other then a bit of a dry cough, there is no reason to not go swimming. If this is a chronic issue, I would suggest evaluation to find the underlying cause (allergies? cough variant asthma? etc). Restricting activity would be for other symptoms as well - fevers, body aches, shortness of breath, breathing fast - then he needs to be seen. Otherwise, watch him and play :)