Ds2 had undiagnosed ADHD for a long time, although he was diagnosed with Asperger's/HFA at 8 and given help on that basis.
These are the things that have helped him, and are on his EHCP (but they could be provided by any school whether a student has an EHCP or not)
Movement breaks (ie give student jobs, like going on errands, stacking books, moving furniture)
Put student where he cannot distract other students by fidgeting
Allow fidget object, or chewy (oral focus increases mental focus)
Very clear instructions for homework, possibly scaffolded
Small group work
Make sure he has understood what he meant to do in class, check he knows where to start
Lots of sport, encourage him with this rather than make it ability or reward based
I found Parent Partnership very helpful.
It is unlikely you will get an EHCP, and you might waste an enormous amount of time organising fighting for this, when the school's SEN budget should allow your child to be supported even without an EHCP. I think the key is to go in, and make sure they look at recommendations from the private report and how they would implement them, and move forward.
ADHD is often comorbid with dyspraxia, dyslexia and ASD, just so you know that the inattention may have followed his difficulty doing the work, rather than his difficulty following the inattention ifysim..ie he is distracted because he doesn't know how to do what they ask, and tunes out.
A lot of ds's symptoms have disappeared the minute he was in a supportive structured setting, with less attrition and detentions. Detentions will not help your son to concentrate, they will just encourage him to think it is not worth bothering.