Yes, all posters saying copyright remains with you unless specifically agreed otherwise are correct - may be worth checking any T & C if you sold your piece through third party, and no idea about US law, which may be relevant if contract not made here (though sounds like it was). So my guess is he is bullying you.
I have seen it from both sides - as a "buyer" always pay more for full copyright than a limited licence to reproduce, and as a producer, get more for assigning all my copyright - and the difference can be very significant.
Agree with posters who say you are undervaluing yourself.
Sad, but true, often people, particularly bullies, take you at your own valuation - if you charge squat, they treat you like squat.
Keep your picture, and tell him to go hang. (Or, more rationally, make it clear that you retain copyright, and he is only acquiring a limited licence - may be worth doing some research on licences, and what to charge for different reproduction rights, and that as per your original agreement he can take it on that basis, or not.)
Good luck.