DMO - I've had this same problem, but ultimately, if the mother does not want to discuss it then you cannot do anything. Do not mention the A word or any word that would give a diagnosis, as you cannot do that and it will just upset her.
You've told her your concerns. Record your conderns as well, but really, until the mother believes or accepts that their child might have slow development in areas, you can't do anything. It's very frustrating, I know, especially as you sound like you really want to help.
I've been lucky and the parents have really wanted my help, so we're all working together now with speech therapists, my network co-ordinator, the health visitor and the portage scheme and it's working great. But it did take a while for parents to open up to me about their concerns and what was happening from their end.
The best thing you can do now is to record your concerns and any developments and do what you can to encourage development at home with him. At meal times when you're sat with them and have close one-to-one time, start repeating a letter sound, then turn that in to a word. It's worked for me so far. Also, putting simple signs to common words used apparently helps to encourage speech.
I HTH