I think it is better your DS gets properly assessed, and help when (and if) he needs it.
He may not need any further support, and it may be a stage, but he cannot be allowed to carry on being aggressive on a regular basis, without the causes being investigated.
He may benefit from the measures a specialist may suggest eg: something as simple as a specific consistent approach, or a place to go when he feels stressed.
Try and see it as a positive move, and support the playgroup.
Unless you think they have got it all wrong, in which case you need to move him to another setting.
But if you think he is currently experiencing difficulties at the moment, I really think it is better for a child to be assessed and supported, than labelled a 'naughty boy' for years before anything is done to try and get to the root of why he is unsettled.
I know this from bitter experience.