Well, it's not like diet soda (or regular soda, for that matter) is a REQUIREMENT FOR LIFE or anything. Meaning, stop if if you want to, and see if it makes a difference in your overall health.
But the "reasoning" behind your son's plea is that there are folks that claim that aspertame is the culprit, that it leads to all sorts of health problems including neurological problems, and so stopping consuming aspertame will create instant health in all sorts of people with diverse health issues.
Whenever I hear these claims, I go to Snopes, which researches and proves...or debunks..the rumors. Here is their report about this:
Be careful ingesting anything "un"natural...
...one of many such articles
as well as,
Splenda is NOT real sugar, as you will read. Be careful. Personally, I don't feel well after drinking or ingesting sucralose products. Good old water may be a better choice. Aren't we supposed to be drinking 6-8 glasses a day anyway? :/ ...Has anyone seen my diet coke? :)