As the parent of a child with mild autism I can relate to both the parents as well as you. As a parent, I was in denial for a long time about my son's condition. Obviously, we did finally have him evaluated and his diagnosis was confirmed when he was 4 1/2 years old. Do I wish we had gotten him help sooner? ABSOLUTELY! If a daycare provider had said anything would I have done anything sooner? I'm not sure...
Here's my suggestion...do you provide a daily sheet detailing his activities for the day? If so, make sure you are specifically addressing the issues that you noted above each day. If they are seeing the description that you provided above being repeated daily, they may take notice of it. Don't sugar coat things! We used to ask the daycare provider everyday, "did he play with anyone today? did he talk more today?" and she would say he's warming up slowly, or he seems more comfortable, things like that. If the parents ask, tell them the honest truth. Lastly, if you feel comfortable just ask the mom sometime "have you ever considered having him evaluated for a developmental delay?" Don't use the word Autism! It scares the you know what out of parents and it will delay them even more. If she is interested in an evaluation she can call the school district to be evaluated by them and then she should also find a developmental pediatrician or child psychologist who can diagnose delays.