Lots of good advice so far. I think you are doing absolutely the right thing in going to speak to the school and wanting to be forearmed. There's a really useful guide for schools on supporting adopted children which includes a handy table listing possible behaviours, what they mean for the child, what might trigger the behaviour in school and what school can do instead. The one I've used is from Essex but I'm sure other LAs do similar. Here's the link www.essex.gov.uk/Publications/Documents/Supporting_Adopted_Children_In_School.pdf
I'm not sure whether an EHCP would be helpful, however, the school should have him on their internal SEN register because he has clear additional emotional and behavioural needs and poss learning ones too.
I'd suggest contacting Parent Partnership in your area. They might be called something else now, but a quick google should find out what and they exist to provide advice / support to parents, particularly for those who have children with SEN /additional needs I think.
In terms of exclusions any FTE needs a re-integration meeting and school must provide this (and give you notice of when it is). As you know, LAC and SEND pupils are most vulnerable to exclusion and as such, schools are required, under the DfE guidance on exclusions, to do all they can to avoid permanent exclusions for these pupils. Certainly, if it did get to permanent exclusion stage the governors review panel should challenge the school robustly on why they are permanently excluding a former LAC pupil and checking whether the school has exhausted all possible avenues of support, prior to the permanent exclusion. But, and it's a big but, the reality is often that if a school wants to manage a child out by way of exclusion they can usually do it.
It you feel after meeting with the school that they still don't 'get it', and are not likely or willing to change the way they treat your nephew, in all honesty I would think long and hard about whether it's a school worth fighting to stay at. is there a way you could move both boys together to a new school? As former LAC he has priority admission, and I think this applies even to in- year applications under each council's FAP (Fair access protocol).
Other than that, if you feel it's best they stay at current school then you need to get the school to fully understand what's causing the behaviour. Your nephew is not consciously choosing to be naughty and they need to recognise that. I'd strongly advise getting an advocate of some sort to help you, e.g. from post adoption support at LA, from Parent partnership, from an adoption charity etc if possible.
The bottom line is that school receives extra funding for your nephew and needs to use it to ensure he makes the academic progress he's capable of. He won't do this if he's too stressed out to concentrate. School needs to put measures in place to support him.