I believe AD/HD is genetic. I have ADD, my Dad and at least one brother have ADD. Pretty sure my Dad's siblings have ADD. Not to mention probable Aspergers in the mix. There's depression from my maternal side, intelligence on both sides and my own quirkiness, developed through genes, the way I was raised and nurtured, birth (9 hour labour, 3rd child), food, all sorts of things. There's an epigenetic theory that many modern developmental issues are related to our grandmothers' diets.

If you're beating him and telling him he's useless, then for sure, feel guilty. But let go that over which you have no knowledge or control. You're not a bad Mum and this is not something you caused. By all means, get him the help he needs but also realise it doesn't help him for you to beat yourself up about it