What records might exist depends on whether your subject was in a British or Loyalist military unit.
Military were mostly formally exchanged, and there should be both some military record of being captured and another sort of record of exchanges of prisoners.These records would mainly be in the British Archives among Colonial Office records in Kew, England.Records regarding POW exchange in US records are extremely sparse, and seldom mention who was specifically exchanged for a POW Patriot held by the British -- where mentioned in surviving material they are usually officers with rank no lower than Captain.
The Mohawk Valley (Ft. Stanwix being toward the western end) had many Loyalists who were not necessarily involved in military organizations, but could have been suspected of plots or were actively supporting military activity in one way or another.There could be Albany or Tryon County Court records of formal charges.There could be mention in the Committee of Correspondence records.
There are also many journals and diaries and other sorts of documents that provide information on events in the Mohawk Valley, such as narratives in applications for Congressional Pensions in the US National Archives, by men who were militiamen and in the Continental and State Troops.Since Loyalists were neighbors of the Patriots who served in these units, they were known and might be mentioned in someone's pension application file narrative (principally the Declarations for Pension).
During and after the war, many Loyalists' properties were confiscated and sold; records of such proceedings could mention your person if he lived in the Valley or nearby and owned land.
You would find it helpful to establish a timeline for the Valley, including military activities.Some helpful places to start would be in the published historical volumes about the Mohawk Valley and Montgomery County.There are also websites that could be of use, both very deep and rich sites:
Also there has been an effort to create every-name indexes from the above-mentioned Revolutionary War Pension Application files on www.fold3.com, a pay site that also has free trial periods.You never know what you might find there.