Yes, he can. It does not matter what he has done. Understand that possession or use of marijuana is still a federal offense regardless of the passage of Amendment 64 here in CO. You are an at will employee and can be fired if you refuse the drug test.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
Yes -- and he can fire you, too, unless you are a contracted employee or protected by some union-based collective bargaining agreement.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.