One place is to shop at thrift shops where the clothes are already well worn. I myself am very sensitive to the way clothes feel, and I prefer only pure cotton fabrics that have not been chemically treated. Even the type of cotton is important, preferably long staple. Even the short fibers can dig into the skin. One of the first things I do is remove the tags that are on the back by the neck. I cannot figure out why manufacturers put these annoyingly rough and scratchy tags in the most bothersome place. If anything, I would rather they put the tags on the side seam, and make them also of cotton, or at least some kind of soft synthetic.

You could also shop on eBay and buy used clothing. You will be doing the environment a favor by doing so and also save yourself a bunch of money. If the clothes are not the latest in fashion, then you could alter them so that they look more handmade or recycled. The recycled look is actually in, and some clothing lines actually try to make their clothes look old or well worn (think especially jeans).

As for the seams themselves, these are very hard to get rid of. Most seams are serged using some kind of polyester thread and the seam sticks out which can be rough on the skin. One solution is to redo the seam with cotton thread, but even this will take time to soften in the wash. I have actually gone so far as to sew on by hand soft cotton fabric to shield my body from scratchy seams and threads. Or you can see if the item can be worn inside out with the serged seam on the outside.

If your son is not particular about how things look, you could take up sewing. People think it is such a mystery making their own clothes but actually a pull over collarless shirt is extremely simple to make, especially if sleeveless. I can make one in 5 minutes on a serger out of a single piece of cotton knit. You just take a piece of cotton knit twice the actual length, fold it over, cut a hole for the head, and serge two seams, one on each side, leaving enough room for the arms. I wear with the seams on the outside. That is the barebones basic. The sky is the limit as to embellishments you can add, of course.

There may be a market for clothing made especially for individuals on the autism spectrum. Perhaps someone can start up such an enterprise.

duenhsiyen

My son's on the spectrum, too, and has sensory integration disorder.

I don't know of any other companies, but he really likes the knit shirts from Children's place, particularly the mock turtlenecks and rugby shirts, because they're really soft. He didn't like one Lands' End shirt he had, but loved another one. Some of his Gap stuff he has hated, but other things he wore until they fell apart, or he outgrew. I honestly didn't realize his problem when he was tiny, because he was almost exclusively in Carter's, which tend to be very soft. Once he was around 15 months or so, my dad started buying him Oshkosh overalls, which he just hated because they were so stiff and bulky. He's 14 now, and still loves his Children's Place shirts. I don't know what I'll do once he outgrows them, which is going to happen soon.

We do go to consignment shops quite a bit, because the clothes tend to be broken in, but still nice.