First of all, start making notes and keeping all paperwork associated with this. Yes, it sounds rather paranoid, but it's a good thing to do. Second, always ask specifically who is observing him, where they are from, and what the purpose is (and add that to your notes). I generally ask if I can meet with them briefly before they observe him so I know what it is they are looking for, and can answer any initial questions up front for them. (Plus that gives me the opportunity to see who it is that is observing him as well)
Is this through the senco then? Did they tell you they suspected SNs of some sort?
If he's having difficulty, regardless of why, I would ask them what they are going to do in order to provide him with support of some sort. Have they recommended that you take him to a paediatrician? or CAMHS? Is he already on school action? Does he have an IEP?
Sorry for all the questions, but it helps give a clearer picture, and obviously we don't want to recommend a bunch of things that are already in place.
Keep in mind that if you suspect he has ADHD, you are well within your rights to go outside the school and get a referral from your GP to take him to a paed and look into it further. There is absolutely no reason you need wait for the school to suggest it.
Sorry, that probably wasn't horribly helpful, but I'm sure more will be along soon.