Okay, well you are well prepared then. If the parents aren't communicating any of this with you, then there isn't much more you can do.

Make your list of things you need him to be able to do before you will get into the training at daycare. They plan this for the summer, so I don't think, (unless you want to,) you don't need to bring it up now. The next time they bring it up, just tell them he is not ready by these criteria... and give them your list. They are free to do whatever they want at home. If they are clearly not interested in your input on his development, there's nothing you can do about it. I would imagine delays that serious have got to be difficult to face and some people are better at it than others.

I can envision a thread 2 months from now, "They brought him in underwear." So, if they are starting without you, my advice is make it clear, diapers (or pull-ups) only - until you say otherwise. Good luck.