I totally agree with wickedwaterwitch's comments about outside assessors. Beware of getting him labeled at this tender age. You could consider getting a private assessment yourself, then letting the nursery know the results, but from what you say about your son's behaviour outside nursery, this course of action seems a bit extreme.
I can see serena's point: if your son sees you at nursery, this will affect his behaviour. I think you need to know in more detail what happens when your son causes problems: what sets him off and how he is disciplined. Can the nursery give you an update each time you collect him?
I speak from bitter experience here: my oldest son was a handful as a toddler and his little brother, so far better behaved, is still on the lively side. I am so used to comments about their behaviour from each and every carer that it would seem odd if I got none! I am now very proactive in asking for reports when I collect them, firstly, so I am in the know, and secondly, to reinforce to the carer or teacher that I will support them over any behaviour problem. I am sure this stops things escalating. Nurseries and schools IME don't automatically assume that parents of a child they deem 'difficult' are that supportive. You have to show them you are.
Having said this, I do wonder if the nursery is a good one. I find it odd that they want to call in outside assessors for a child of your son's age, especially when you have told them that he plays well outside the nursery. Are you confident that the carers are sufficiently skilled? Have you seen the Ofsted report?
Keep posting here and let us know how things develop.