It is never too late to toilet train and there have been many approaches that have been shown to lead to success. I've worked with developmentally delayed persons over 20 years old who have learned to use the toilet independently. Looking for a Board Certified Behavior Analyst through the BACB certificant registry (www.bacb.com) may help you identify a behavior analyst who can help guide you through the process. One of the most cited approaches to successful toilet training is Azrin and Foxx's "Toilet Training in Less Than One Day." This is a book that provides detailed information about how to implement toilet training and it is available for purchase at multiple online sites. I bought a copy for my cousin at Barnes and Noble just last year, so it might be available at your local bookstore.
Another source of information about this technique can be found at:
http://nationalautismresources.com/azandfototri.html
Hi! my son did not learn to tell us before the act either. after a lot of trying everything we gave him what we call a "bell-trousers" at night. Its a device that makes a loud noise with the first drop of wetness and is supposed to wake him. It requires getting up like with a baby and took a few months to have it sorted. He initially did not wake up so we had to raise him and physically lift him out of bed and sit him on the loo (dad had to do it as he is too heavy for me). and IT WORKED!! we never looked back. after nights had been sorted day came along almost by itself. OUr doctors did not know why but allegedly there are kids that sleep too deeply for the apropriate hormones to trigger the urge. there are also psychological issues of course. anyway it worked for our son. maybe it works for you. keep my fingers crossed.
I would get a professional involved who is a going to put together an approach that all the family sticks to. The most important thing is routine and consistency, so everyone has to be on board 100% of the time. The consistency needs to be down to everything as very small changes to the environment, routine, words spoken as prompts etc can render it a whole new experience for him and he won't recognise it as the same thing. An example of this is that about a year ago I went to have lunch with my son in a cafe. He wanted fish fingers, but the waitress said "they're not called fish fingers anymore they are called fish nuggets". That really upset him because he didn't know what "fish nuggets" were, eventhough he knows what chicken nuggets are, he couldn't use that information to guess what a fish nugget might be like. He got very upset and was lying down on the floor screaming, whilst I was waiting at the checkout!! And my son is high functioning autistic who goes to a mainstream school and was potty trained when he was 3. So, if that small difference in language upset him so much, you can imagine how other changes you three adults make could upset him so much that he doesn't understand what he is expected to be doing. Does that make sense?
I think the professional you need is a behavioural analyst and one who has experience of autism. But all three of you need to sit down and agree on how you are going to do this. It might be that only one person initially works on the programme and once he has learnt it it is then generalised out to the other adults. That might make it easier and quicker for him to initially learn rather than having three different adults. But obviously that is going to mean alot of work for that one person. I would discuss your options with the BA.