I agreed with PatriciaBaca. Autism is for life, so the "diet"/ dietary lifestyle is for life. Changing that at any time will only result in regression and a negative change in behavior. Yes, he should continue with his meals and dietary needs.

@Should an Almost 18Year Old Stop AUTISM DIET........ The Answer is YES .if you want to continue to see IMPROVEMENT then CONTINUE what you Know is the Right Diet, Suppliments and Treatments .........If Your SON chooses NOT to CONTINUE these then expect to see Regression and a Big Changes in a NEGATIVE Way.

Sorry you are sister not aunt! X

Yes, if it is working and you all believe it has reduced behaviours and traits that needed culled, keep on doing it. I believe it's a change for life! The diet is as important to the brain as the gut!
My son is only 3.5yrs old, raisins and grapes have him up chilling his body in pain he cannot digest them, they blocked his bowel and through process of elimination we took them away and now he is much less susceptible to this horrible backed up bowel, bad gas, overflow scenario which saw us do hospital dashes at 3am! Good luck.
If it's not broke dont unfix it Auntie 😉