In the given context you should say "he was a leader."
If it said he was leader of the battlefield surgery and wound care squad that would be perfectly acceptable. But, it is speaking of an abstract in the form of a technical field of study.
The other form you've asked about "He was the leader", has a different meaning in context. This would mean that of all the surgeons he was the single best in this field who came up with all of the major advances on his own.
Compare this to "he was a leader". Which means that amongst the surgeons he was one of the best in the field. One of the men responsible for the major advances in the field (but, there could have been others).
As you see, shifting the article from a to the has a dramatic shift in meaning.