Reverend at the Church-of-England chapel in Amsterdam around 1764.
Probably the John Charles, D.D. who is listed as chaplain of the Church of England in Amsterdam in 1741.1
Life with Boswell:
Boswell went to his sermon on May 26, 1764, describing the sermon as good and Charles as "a very pretty man".
Note 1: Listed in the Rev. William Steven's "The History of the Scottish Church, Rotterdam", p. 282 (find it at AbeBooks.com)