Artware Fineart has dated this Georgian Gentleman Oil Portrait to the 18th Century.
Portrait of a Gentleman
oil on canvas
10.1/2 x 8 in. (26 x 20 cm.)
The high, turn-down style of collar was fashionable for this period, although not a young man the sitter appears to have abandoned the traditional wig and is "wearing his own hair".

With thanks to Dr Susan North from the V and A for her help in identifying the clothing.
Francis Alleyne was a British portrait painter, usually on a small scale. He seems to have specialised in small, oval, three-quarter lengths, usually signed on the back, which look like middle class versions of Wheatley's more elegant small portraits.
Artware Fineart has dated this Georgian Gentleman Oil Portrait to the 18th Century.
You can view the stock and full dealer information on the Artware Fineart Profile Page