glue ear links with ADD

My nephew was diagnosed at 3 to have glue ear, this did not clear up as he got older and since then he has become partially deaf - he now wears hearing aids and he is only 7.
His behaviour is affected because of his lack of understanding due to loss of hearing, but he also has poor language skills as he never learnt to pronounce letters propperly.
His school is refusing to test or even moniter him for signs of ADD (which he has many!) as they put all of his behaviour down to his hearing loss.
Also he has a half brother with ADHD.
How do we go about getting him tested for ADD or at least trying to get the school to listen to us.
thank you