I'll make a bold statement and say that I've never run across any vape gear worth buying that was draw-activated. All of it that I've ever seen was those tiny 510 batteries that a lot of people used about a decade ago. Obviously there are types of gear that I'm not familiar with, so maybe someone else can give you better advice.
The combination of "tube style" and "adjustable wattage" will limit your choices. There are lots of great tube mods, but they are mostly mechanical, and you control the vape with the choice of coil resistance and the level of the battery's charge. Off the top of my head I can think of just a few candidates: Tesla Punk Nano (86W variable wattage tube) and I believe the Nunchaku tube also. There is something called a "Magic Wand" that I've never tried, but I think that those are also variable wattage. All three of those devices are button-activated, but they do use external batteries. I am fairly certain you can still buy a Magic Wand at some of the Chinese vape gear sites, but not sure where you'd find either of the others.
Does bro want to start vaping, or are you trying to convince him? Just being nosy, don't feel like you have to answer, but I think many of us are familiar with being in that place where we want to make vaping as seamless as possible in order to get folks off cigarettes, but we're also familiar with the 1,825 excuses those people can make as to why vaping is just too much trouble, or they just don't like it, or they feel funny having other people see them vaping, blah blah blah. I went through it with my own sister, who was diagnosed with COPD when she was in her early 40s and wouldn't stop smoking for anything. She was disabled and ended up coming to live with me (under duress, she had no other choice at the time). We went back and forth for about three months and then one day I just told her I REFUSED to keep buying cigarettes for her so she could smoke and die a little at a time living right up under my nose. The hand-holding that ensued was stressful, but she made the switch finally, and she is still vaping and not smoking.
Good luck to you, and good on you for trying to help your brother. Just don't kill him if he's a PITA for a while.
You sound like a good brother.