Help to identify a plant please

wonder if you can help me identify this plant which is about 5 foot tall growing in my son's greenhouse. He moved into the house a year ago and we have been unable to identify it. There have not been any flowers or fruit on it. At first we thought it was a walnut tree but its leaves have stayed on it over winter. It is not a heated greenhouse.
Thank you for any help you can give.

Click image to enlarge

Re: Help to identify a plant please

Reply from Janet Lacey

feeble guess - but is it an avocado? Though, would have thought it was too cold.