Sorry, I have no solid suggestions, but I enjoyed your post. This all is just kids being kids to me, having to learn responsibility. And possibly an indication that you were right the first time thinking he wasn't ready for a phone ... but knowing there were good reasons you overrode that feeling, I would be patient, pick your battles, and give him a little time to adjust. For me, handling these things lightly (gentle joking) always worked best, although sometimes a back up threat along the lines of "if you can't handle these requests, maybe you aren't ready for the device" is helpful.
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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).