A little out of the blue, but has anyone brought up KR808s?
I ended up in a conversation a few days ago that brought my attention back to them; I haven't used them in 2+ years, and apparently they've advanced a bit since then.
Since you (OP) emphasized wanting simplicity and something smaller than an eGo, something like a
classic VK starter kit might do nicely. The prefilled-carto/battery 2-piece kit is super easy -- unwrap carto, twist onto battery, vape until dry, repeat --and blank KR808 cartos can be found pretty easily when you're ready to strike out into the vast buffet of flavors available. There are tanks and mods available if you want a bit of an upgrade. You can get a carrying case that charges from the wall and then charges your batteries while you store them. They have passthroughs that run off a USB port so you don't have to use up your batteries to vape when you're at a computer, a wall charger near the bed/couch, or what have you.
Normally, I'd recommend the 510-to-eGo/KGO route, too, but for absolute simplicity, the V4L stuff and the others like it shouldn't be overlooked. The upgrade path is more limited, and these aren't simply interchangeable with the 510/eGo platform; I don't know if you can get as far into low-resistance atties or variable voltage and the like as you can with the 510/xgo stuff. But if you find all that confusing, intimidating, or just plain annoying, and all you want is a nice vape, the KR808s might interest you.
Technical stuff: I think their batteries run about 600-900mAh (please correct me if I'm wrong, someone!) so the battery life is comparable to your son's eGo, unlike the mini eGos that will only give you a couple of hours. They're a little thicker than a cigarette and can be a little bit to a lot longer, but they're fairly light and honestly kind of elegant. I still have some I decorated with nail decals to use when I'm playing dress-up on the town. I don't know what the voltage on them is, but the throat hit and volume are comparable to a mediocre eGo and superior to the average 510.
The main drawback is that buying the prefilled cartos gets expensive. Maybe not quite pack-a-day analog habit expensive, but nowhere near as affordable as vaping can easily be. I guess that's just a matter of how much the simplicity and no-fuss features mean to you. Corollary to the main drawback, because you mentioned wanting something refillable, is that refilling the cartos can be annoying. For that purpose, I'd tentatively recommend a
tank; you just squeeze-bottle or eyedropper the liquid into the big open area of the tank (not the center part! except maybe a few drops on a new, dry one) and you're good for a few hours, even with heavy vaping. I say "tentatively recommend" because while I've used equivalent tanks on eGos and found them quite serviceable, I haven't tried the KR808 version yet; mine should be arriving soon, because I'm curious to try one out and see if I can bring my "classy" batteries back into play without the expense of prefill-cartos or the fuss of refill-cartos. The one I linked is a dual-coil, low-ish-resistance cartomizer, which basically just means that it should perform quite well, comparable to your son's, but might drain the battery a little quicker than the cheaper prefilled cartos.
I hope you find the thing that makes you happiest! There's a lot of variety in personal taste when it comes to vaping; fortunately, there's just as much variety from the companies trying to satisfy us!