Nice matching of perspective and sizing. Could make it more convincing if you added some "painterly" effects to the body.
Good attention to light and shadow play. I would only suggest running the photo source through filters to give t a painting texture.
Neck seems a bit odd in the back where it touches the collar, compared to his right side.
You have the wrong Rank on him. Washington is a six star general and I know that you are going to say that there is no such thing but in 1976 Congress created the rank: The General of The Armies of the United States so that no general can ever out rank Washington.
He should also be in an Army uniform as opposed to a Marines Uniform.