Nintendo switch has a parental control app that can help you with the whole time limit thing
switch is both tv and handeheld, so not quite just a handheld, but I can understand your thinking as the handheld feature is the most advertised thing about the console
Nintendo is now investing into more adult games like doom if that is what your concerned about. not to mention it all boils down to your kid's personal tastes, if he's like me and loves platformers, rpgs, and puzzle games then it's unlikely he'd outgrow nintendo. Sony has many great games from what I've heard too, so the games will be outdated hardware wise, but still considered great in the next few years, like how mario 64 is.
For me getting a switch was for nintendo exclusives, but I'm sure people will do the same thing with sony. this is the reason why xbox one didn't do so well, too little exclusives. I'd say show your son some games that are appropriate for his age group on youtube from each system and let him decide what he wants.