He says he has taken the money to buy snacks when he is out with his friends on his bike
Do his friends have money for food and does your DS get any pocket money?
junk food mainly - which I obviously don’t buy him, he is given mostly healthy meals and snacks and when he says he is hungry is offered yoghurt, fruit. He is already overweight, which is another issue.
It's pretty normal for someone with ADHD to be Dopamine chasing by eating junk food or even sensory seeking by eating or drinking often.
When my DS was eating constantly I found that what we were giving him wasn't quite in the right balance.
Have a look at this guide from the Caroline Walker Trust. We read the one for teens when DS was overeating, made a few tweaks, and his overeating did cut back.
Is he on medication for his ADHD?
He is already overweight, which is another issue.
Does he do any other activities rather than go on his bike? Things like a Martial Art, Cubs, Athletics, Swimming or Hockey might be really good for him.
Could you or your DH start the C25K with him or do workouts at home? He might start getting his Dopamine from exercise rather than eating junk hopefully.
When my husband caught him going through the drawer earlier he denied it and continued to deny it, then admitted to taking money once, and promised he hadn’t taken more, then it was twice, now three times. I honestly think he has no idea how much he has taken.
My husband thinks we shouldn’t buy him birthday presents for his birthday in October but I feel this is just too cruel, I couldn’t do that, but at the same time this isn’t just 50p, £1 here and there it’s £20 notes he has taken. We are struggling with what to do as a way of consequence for this.
I wouldn't give him a delayed consequence like this either. With ADHD he's likely to have an inability to reflect and also, as I mentioned earlier, a level of immaturity. What you do though I'm not sure, I'd maybe post in the SN Children Section to ask for suggestions. What I would say though is that if you are going to punish him, it needs to be immediately.