Start the potty training after the cast is off and he can walk pretty good and sit without trouble. Once they realize they shouldnt just wet their diapers/pants, they start to hold it better in their sleep.(it could be months) In the daytime when he says he needs to go, you will most likely go into the bathroom with him for a while. So in the night if he should happen to wake up and need to go, he will call for you, and you will get him up and help him, and then put him back in his crib. After a while when he is able to go without help, you will then know its time to put him in the toddler bed so in the night he can do this all by himself. My grand daughter is 4 now, and still yells for someone to come with her to go potty in the daytime, and has only woke up once in the ngiht to go. She hasnt been in a crib for a year, but staying dry at night and not needing help would have been the big clue to get her out of the crib if she had still been in one. You dont need to hurry with the bed yet, you have too much going on with all youve been thru lately, and dont need one more thing added to your plate right now. Once he can go potty independantly you can then switch the beds.