It is a shame that the nursing home hasn't noticed before you. I just worry that , due to the brace ill placement, he may have a sore either on his leg( or foot) or like you said a few sizes to big and is causing a sore/blister. Isn't there an administrator you can bring this up, since no one else had noticed? I'm not trying to sound frantic but I just worry for I worked in NH at one time. 

Ray had to have new brace when he lost weight as the old one moved awkwardly resulting in a ulcer on his heel that took a long time to resolve. These things happen partly because of the little time staff have to really observe the patient, when a shower is a splash and dash affair. Ask staff to please look at skin condition etc when they shower him. You take time for yourself too my friend.

If the AFO is causing pain then he won't "trust" the leg to hold him and is probably not putting any weight through it so then it buckles as he's subconsciously not turning on the muscles. His body is probably trying to limp, but he doesn't have the control to do so.  A PT assessment and/or an orthotist review are needed. Good luck with getting the Home to do what is needed.

My best of luck to you on receiving what seems to me a necessary. I wouldn't just take no for an answer. I so agree with the above posts. I would be worried about his skin and tissue integrity. You keep fighting. Good luck to you both.