@mitzi3 We have used tshirt fabric for ties, very soft and they seem to hold well on the back of the head.
Are you referring to a homemade mask or one provided by the clinic?
If you must use a clinic-provided mask, I suggest bringing soft fabric -four 1" wide by 10" long strips of an old tshirt or 4 soft, fluffy peices of thick yarn, plus a needle and thread, and sew them in place of the elastic.
If you can furnish the mask, I suggest a soft old tshirt, cut in the shape of a mask about 3-4" high and 5-6" wide for a 4 year old, with 8-10" long ties cut right on the 4 corners. Best if you can double the fabric, and be sure to cut across the shirt so the stretchy part goes around the face for comfort. No pattern needed, if you are moderately handy with a needle, gather it along the two sides to about 2 1/2" wide so it fits his face better.
I second Colleen's suggestion to put one on an animal or doll, or yourself, as well – especially one just like his.
Finally, as with all things with an autistic child, practice wearing masks together until he is comfortable.
Come back and ask if you have more questions.