First, a quick legal lesson. A retired game warden may be still commissioned as a special warden. This means he is still an active peace officer with full peace officer authority. This is also true for retired DPS troopers (called special Rangers) and retired TABC agents (called special agents - we have 94 currently, IIRC).
So, knowing this, in answer to your questions about who to notify, I would call the local game warden AND the Austin Office of Professional Responsibility. The local game warden handles the investigation of hunting problems while the OPR handles the complaint if it is a retired GW. I would probably also call the local Sheriff but do it more as a courtesy and let him know I had already notified the game warden.
Now, as for how to handle the situation. I first want to say you handled it well. You came away with no injuries or unnecessary shots fired and were smart enough to keep your nephew out of danger. I am assuming you were able to finish claiming the deer and recovering it for him, of course. You had the option of being friendly or being standoffish and you chose friendly. I could never fault that decision. It was not required of course, and you were not required to answer any questions until he properly identified himself. In this case, I think that type of attitude would have set him off worse. You might have asked him for his ID and verified it because you can do that with any peace officer. But, I have no better way to handle it than what you did.