He's not concentrating, again

Hi, we are new. I’m Mom to Twins and my one son has ASD and the other recently figured out ADHD. I feel terrible as we didn’t realise bad behaviour was symptoms and he’s back from boys town and doing same old things. Oddly on autism forum many have new dual diagnosis with both so it helped me confirm suspicions. I don’t like drugs and I’m thinking of trying l-theanine and maybe Gaba, if anyone tried it let us know.
Guess I’m just trying to better understand ADHD and see where I can help him. He’s bunking homeschooling, rearranging my house rules and running around all day with lack impulse control, luckily the stealing has stopped, that was why I worried and sent him away.
I am celiac and hence we have most family on this diet, my ADHD son seems to have Candida overgrowth symptoms so I have him on strict no sugar/glutton but he walks in eating chocolate so it’s just patience running out.