Sometimes dogs learn something and it isn't what we wanted. If you have him sit and then say stay and he does something else (like get up), he may have learned that stay = get up. I know there are some trainers that do not even use the command "stay" any longer. Once you put your dog in position (sit or down), they should hold it until you release them. I think Bronx believes, "stay" is his release word

I would forget using the word 'stay' and come up with a release word you will remember to use each time. Don't use "Okay", as you will be surprised how often we all use that word in normal conversation Try something like "All Done" or "Finished" and put your hands in the air to help get the message across.

It would be something like "Bronx sit." He sits. Give him a sec and say "All done" raise hands in air and cheer him for such a good boy. You should be able to lengthen that wait as time goes on. Good luck!