Based on the facts as you present them here, it doesn't seem like you've broken the law in any way (other than not registering the dog..). However, that doesn't stop him from calling police or suing you in small claims court for possession.
Thomas A. Schaeffer, Esq. Law Office of Juarez and Schaeffer PO Box 16216, San Diego, CA 92105 (619) 804-4327 www.jslaw.org This posting is provided for "informational purposes" only and should not be relied upon as "legal advice." Nothing transmitted from this posting constitutes the establishment of an attorney-client relationship. Applicability of the legal principals discussed here may differ substantially in individual situations or in different States.
Tell him that if he wants the dog he is going to have to sue you and if he sues you, you will counter-sue for all your expenses in caring for the dog (food, vet, etc) as well as a daily boarding fee for the time the dog has been with you ($25 - 35 per day). Tell him he is not to contact you again unless it is through an attorney and if he does you will go to the police and file harassment charges.
If we do not have a signed fee agreement I am not your attorney and this is not legal advice.
Yes, you do. He gave the dog to you. He will need to file a lawsuit to get the dog returned to him. Registration is evidence of ownership, but not ultimate proof. Tell him that you will charge him for each day of the dog's care if he files a suit against you. Then tell him to stop bothering you or you will file for an antiharassment order. If he continues to bother you, consult with a local attorney.
This is not to be construed as legal advice. I do not have an attorney client relationship with you.