I've been a volunteer firefighter for a decade or so, and was motivated by a lot of the same things your kid is. I grew up farming (I farm now full time) but I was practicing law full time when I started doing and I was motivated by a desire to help out in the community and get back to doing some honest to goodness real god damn work. Farming full time keeps me away from a lot of calls, but I plan to do at least another 5 years, and have found the experience to be extremely rewarding. If I was doing things all over again, I would strongly consider telling young Al to get into full time career fire service.
But to @BabaYaga's point, you will see some shit. I'm just a volunteer and I've seen plenty of dead bodies (we respond to all car accidents in our district). The worst thing I was ever a part of wasn't even a fatality though - it was a lift assist (that's when EMTs need more man power to physically remove a sick or injured person) on a bed ridden 500 lb man who thought he was having a heart attack. We go into the house, and all you could smell was BO and shit. There were pizza boxes everywhere. He was in bed naked, and shitting himself into a bed pad. His feet were swollen and purple and the skin was peeling off. I actually had to run out the front door and vomit into the bushes before I could really help carry him to the ambulance.
I told my chief later in the week that that experience (of seeing someone who was alive but clearly decaying) was harder than any fatality I'd responded to. He agreed.
So, yeah, buckle up, but I doubt he'll regret it and this may be his calling. Your kid sounds like a badass, IMO.