I would start a dialogue. You could really make a difference for this child.
My son had a friend that I thought might be depressed. I was in a quandary like you. We were talking and she confessed some difficulties with her son. I finally got brave and told her that her son reminded me of the daughter of another friend of mine. That the daughter was depressed and that she got on medication that really helped her. I just said that an evaluation might be something to consider. In the end she did just that and really helped her son.
I understand it's different for you because the Mom is not confiding in you. I would still start a dialogue. You are a babysitter and have some experience with children. Is this child an only child? Maybe the parents don't have anything to compare to.
You could start with, "Your son is so easy to take care of, but sometimes I do wonder if there is more I could do for him....." Somehow start a dialogue. The Mom should ask what you mean at that point and you can tell her about the things that seem different than other children you have taken care of. Let her know you wonder if an evaluation and early intervention could make a difference in how he socializes. I wouldn't throw out terms like "autism."