It seems that you do not have any records telling what his rank was. This may be important when getting a uniform. It was not uncommon for assistant surgeons in CS service to be enlisted. Without doing some specific research, I believe I have read that some assistant surgeons were not even med school grads, just sort of an apprentice to the surgeon.
If you determine that he was an officer, he would be responsible for obtaining his own uniform from commercial sources. I would suggest you research what other surgeons were wearing during the time frame and location in mind. This will give you a starting point. It may be that the typical uniform was the same pattern as a regular line officer. Also if you want a quality uniform made of period type material, construction and pattern, be prepared to spend some money.