Look on the corner of the table, between the boy's hands. That is a flying squirrel, and this is John Singleton Copley's (1738-1815) Boy with a Squirrel from 1765.
The boy was Henry Pelham, Copley's half brother, and he must have known he was posing for a piece in which his brother was placing much hope. Copley planned to send this piece to London's Royal Society of Artists exhibition. He did so, and it paved the way for his solid career and his eventual move to London, where he stayed the rest of his life.
Flying squirrels were often kept as pets at this time, as they were friendly and docile. Note the little creature has a delicate gold chain to keep it from getting away; but note too how open and gentle the boy's expression seems even in profile, and how light and tender a grasp he keeps of his tiny friend. Copley was famed for this ability to say much about his sitters' selves with only a few objects to help. However, the painting for which he's most known is about the grand struggle between man and nature: Watson and the Shark.