Go slow and careful with the meds; sometimes they are actually the cause and not the cure.
Pay attention to his sensory needs because they may have changed. We know, given that he is 13, that lots is changing in his body, and he is probably confused by much of that. And those changes can and will bring about changes in his sensitivities of every type, so pay attention and look for triggers, just as you probably did when he was very young and you were new on this journey. But things change ... don't forget to look for the changes.
Sorry to hear you are experiencing a set back and best of luck getting it resolved. And, yes, just as fast as new issues seemed to develop, you will probably magically find the solution - but no one can tell you how long that will take or even offer certainty it will happen. Trust your instincts, pay attention to your child.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).