Hello,
I work in sales and customer service for a living so if it were me, I would offer him 2 options (Options are always nice) what the terms of those options are is up to you but something along the lines of 1. pay the remaining balance of the snake by X date and I will ship that day, or 2. I will refund the money you have provided and sell the snake to someone else.
Of course its best to dress it up at the beginning, you know like, Dear Sir, Following up on previous email, I understand life gets busy and out of consideration I am emailing you today to in hopes we can finalize our transaction. With weather changing blah blah blah, I require response from you by end Oct X (whichever date you prefer) in order to ensure a smooth shipping experience for both you and the snake. In order to be fair to everyone I would like to offer you a chance to finalize our deal provided you can send the remaining balance by the date stated above, or contact to me to make alternate arrangements. If you are unable to contact me by the date above I will be forced to return the money you have put forward for the animal to date and offer him up for sale once again.
Something like that. Give him a choice and let him know your position and try to be fair to everyone. At that point the ball is in his court and you can make whatever decision you have to without feeling a lick of guilt. You offered the chance.
Hope I could help.