Hi, I'm not familiar with UK laws. But very familiar with US educational laws. I have 3 children all with special needs, they all have IEP's (individualized Education Plans) now. But it wasnt easy, the school did not want to put out the extra services. For us, Special Needs were adhd for starters with my daughter who's abstract though ability was amazing, but her boredom with mundane tasks lacked focus, and a visual processing disorder for both of my sons who both tested highly with abstract thought and problem solving, but the actual reading and writing were difficult. So now my children apply for extra services, and special ed is a normal happy thing now, everyone is unique and learns differently.
If your son is having issues being naughty at school they should help you with that. His self-esteem is at risk. i wish i would've been more of a squeaky wheel with getting my daughter help earlier on because you are exactly right, he could be at risk for drug use and depression. My daughter even tried to hurt herself at age 14, then they finally listened and gave us the IEP to get her help and counseling through the school. It was too late to save her education at that point, she already hated school so much and knew that she'd been treated poorly by people who were supposed to like kids and want to help and teach them. But not too late for you and your son, you are getting a head start on it. My 10 year old is doing great, i make use of all the resources I can to give him every help possible.
I think you will have to learn your rights for your area, and insist they do testing, and get him help he needs. By the way my son scored so high on the Gifted and Talented Program, in 3rd grade, and I thought awesome, finallly he will have cool and interesting new experiences at school. But nope, they even told him before they told me that because of his poor behaviour he would not be allowed to participate, and that there were many other kids who would know how to behave in the special program.
Good luck, act now, and learn your and your child's rights of education in your area. Please know what a fabulous parent you are!!