Some autistic people have joint problems due to problems with their methylation cycle not producing enough cysteine. Cysteine is a very important amino acid that the body uses for many different things, but one of them is as a source of sulfate, which is important for repairing the joints. Any person who has both joint problems and autism needs to take methylsulfonylmethane (MSM), an over-the-counter (at least in America) nutritional supplement that is easily converted to sulfate. I cannot stress this enough! I personally know multiple people who have benefited from daily MSM supplementation and would not hesitate to strongly recommend it to Mr. Enter, or any autistic person with joint problems for that matter.
MSM is often packaged as a combo with glucosamine and/or chondroitin, which are also said to help with joint issues (and I've personally benefited from this, so again I would recommend it to anyone whose joint issues are not completely solved with MSM alone). In fact, it might be worth taking even if you are autistic and don't think you have joint problems, as if you are used to your bad joints you'll literally never realize that it's abnormal to have joint pains for days after every workout or from having a job that demands standing for hours.
Also, by freeing up cysteine from being broken down for sulfate, you'll indirectly boost your cysteine levels and thus increase other important bodily chemicals that are deficient in autism, such as taurine (important neurologically and for digestion) and vital antioxidants such as alpha lipoic acid, glutathione, and sulforaphane (again, these have important neurological roles).