Bunni..I am so sorry. I don't think my answer is going to be a popular one. Unfortunately I don't see this getting any easier. I am also a nurse and I have been there, that situation where the work gets ahead of you and you look up and see the time and go "crap...I was supposed to leave 2 hrs ago". I have in fact started setting my phone alarm if I have to be somewhere at a certain time. Sometimes it is not easy to walk away and make a call or to excuse yourself from a patient or family.
Is he able to carry his phone with him on vibrate? Perhaps if he can you could text him at the end of his shift just to remind him of the time, or if he has a smart phone you can at least use the find my phone app to check his whereabouts. I also found an app called avocado for iPhone users that is great. It has a GPS setting so you can see where he is and you can send kisses etc.
I know this seems to be putting the responsibility on you as the BS, but I think in this situation it is going to have to be a two way street to success. (((Hugs)))