I always sit differently on this topic as I refuse to use pull ups. They aren't stupid these kids, they know they are diapers. This child is older so a different approach to toilet training needs to be taken in my opinion. I would actually hazard a guess that conditioning him would help in this case. Take him every 30 minutes to the bathroom and gradually increase the duration in between visits. I totally appreciate those of you who say that's you don't have time to take a child to the bathroom every 30 minutes but I think you do. The rest of the group gets neglected? Not really, your routine becomes much more "free" rather than structured activities so the kids can amuse themselves while you step out for what, a minute each time?? In a couple of weeks I think you will see improvement and none of the other children will be any the worse for it. It is in the child's best interest to deal with this and yours too, to get over this difficult time. Hard work yes, but for many different reasons children are more or less demanding of us throughout different stages of development. After a week or two of conditioning it will become habit and you can at the same time work closely with him teaching about how to verbalized his needs, talking about holding his bladder through books and diagrams and all the while take some pressure off of him because you will be taking him frequently to the bathroom and as I said increasing the time between visits after a few days.
This is not how I would normally train a child but at this point what you are doing isn't working and the parents are not being helpful and the only person this is detrimental to is the child and in addition is a pain in the ass for you to deal with. Lose the pull ups and put underpants on him. Conditioning should mean low chance of accidents because you are telling him to pee not asking him to, and reward with stickers for all his success'. Everytime he pees ask him what he should say when he needs to go. Tell him what it is and have him mirror it to you. He is obviously not capable of controlling and verbalizing his toileting so just work on one at a time.