I think you are on the right track with getting him immediate help - kudos. I know that is hard to do. Now, you are going to need to press as hard as you can to get him in for full comprehensive neuro-pysch testing ASAP, because as others have mentioned, no one can diagnosis your son in an hour. In addition conduct disorder, is one of those disorders, that cannot really be diagnosis until over 18.
I have no doubt he has ODD, and ADD - in fact your son sounds a lot like mine at that age - but there was a lot more to his diagnosis than just those 2 things. A LOT more, and it only escalated as he got older.
What medications is the Dr suggesting? Because if it is a stimulant medication than I would refuse until you get him a full work up. A stimulant can take those ideas in his head, and cause him to want to act on them, unless he is in no other way mentally ill.
The other choice you have is going to your local children's psychiatric hospital, tell them everything that has been going on that seriously listed - and they will probably admit him. Especially since it is summer, now would be a great time. If he is admitted, than you get him all the services he needs in one place, a diagnosis, and some medications, and get on a better track for the new school year.