Hi Purplehouse,
I'm afraid that the artist owns the copyright in the painting, despite being commissioned by you, and that allows him to sell copies etc of 'your' painting. If it had been a commission to make a photograph for private and domestic purposes (such as a portrait of you to give to your husband) you would have had some protection under Section 85 but that doesn't apply to paintings.
However I agree that, ethically speaking, he ought to have sought your permission. If he is a member of the Association of Animal Artists, you could try taking the matter up with them. http://www.associationanimalartists.co.uk/
Sorry not to have had better news for you.