If you really think you have Cushings, you can ask this doctor for a referral to an endocrinologist but I don't think it would be worth your time, energy, or money to try to talk him into testing with the response you've already received. Even if you "won", chances are really good that he wouldn't order the right tests, wouldn't order enough of them, and wouldn't know how to read them either, so you'd still be where you are now or worse, he could falsely "prove" you don't have cushings making it harder for you down the road. And honestly, the same things can and do happen with endocrinologists too. Those who can afford it (me included) skip go and travel to see Dr. F, an endocrinologist and cushing's expert, in L.A. www.goodhormonehealth.com It can be reimbursed depending on your insurance, you don't need a referral to see him (for his sake, you'd have to ask your insurance if they require it), but most people have to travel and you have to pay up-front for the appointment because they don't bill insurance.
Feel free to join the forums and ask more questions. I hope you get the help and answers you're looking for!