@larrysamiam Hi lovely lady! Just thought I'd jump in here! As a primary teacher I am quite adept at supporting parents in getting help with this kind of thing.
1- contact GP and make an appointment.
2- write down everything that is going on. His impulsivity, stealing, lack of sleeping, inability to regulate his food intake etc. This will help you to remember what you need to say.
3- Contact school and ask if they have noticed anything in his behaviour there. Speak to pastoral manager for his year group, form tutor. Arrange a meeting if necessary to discuss what your concerns are. You can ask for them to write a supportive letter to the GP detailing what they have seen.
4- Insist on a referral to a paediatrician. The GP shouldn't refuse as they can't make a diagnosis only refer on.
5- Try and keep a diary of when he does these things. Is he taking at certain times of the day? When is his sleep most difficult? Might be able to see a pattern which might help you to plan for prevention.
Please be kind to yourself. I would certainly be careful with money, cards, expensive stuff as this stops bills etc being paid. With food, I'd try buying less snacks etc and lots of cheap bread and cereal (I have teenage boys)!
You've got this. Some of this is because you are the safe space and he might be holding in the urges when out and about. Good luck!