You need to talk to your daughter about this.
You do not say whether she goes away as part of her job, or for pleasure, but leaving a young child, which I am assuming we are talking about here so often is not good, and leaving an autistic child is even more risky.
Please do not ignore the boy when he calls you Mama, simply say, I am Grandma, dear, not Mama. Any child under the age of seven or thereabouts could be forgiven for calling you Mama, as it sounds as if you are doing more mothering of him, than his mother is.
Sorry, if that came out wrong - I don't know the circumstances, it sounds as if she is a single parent, working full-time, so she may well not have much choice in this matter.
Would it be possible for her to change her job, so she did not have to travel as much? Doing so, would probably be better for the child.
I hope you find Gransnet helpful. If I may, I would suggest you give a little more detail another time. As you can see, some of us have difficulty offering advice as we do not know whether your daughter is a single mum or not, or why the child's father apparently is not doing his share of child-care.
If your daughter's many absences are not work-related, I suggest you take her severely to task - it is fine that you help out, but not to the extent that the child is confused as to who is mother and who is grandmother.