I was in the Marine Corps so I'm partial to recommending that branch
I was attached to the Air wing and knew a couple guys who did Fire Rescue on the flight line. Got to go out with them on some swift water rescue training with the L.A. fire department and also some major controlled burns where they walk into walls of fire to put them out. Cool stuff. Don't worry, they are all geared up and safety is a priority.
be VERY, VERY careful with the recruiters.
I'd like to second what Sad in AZ said. I'll tell you what happened to my BIL to illustrate. My BIL wanted to be a Military Policeman. the recruiter can only guarantee a general field. Within each field there are multiple jobs (aka MOS). My BIL said ok. Instead of becomin an MP, he was assigned to corrections, basically working with prisoners in the brig. Didn't like it at all.
The recruiter may be able to provide a general job area for your DS, but can't ensure that your DS will get the exact job he wants. Be prepared for that if your DS decides to go that route. It all depends on what that particular branch's needs are for personel in each job. If there is a shortage of MP's and your DS wants Fire/Rescue, and they are both under the same general field, then your DS becomes an MP. The recruiter will make all kinds of promises and claims that your DS will get what he wants. But don't buy into it.
I am speaking from experiences I saw/had in the Marines. Army, Navy, and AirForce may have different approaches. However, the recruiting aspect would still be the same. Hope that doesn't completely discoursage you but its something you need to be aware of.
FYI - I have a friend now who went to the FF program at the local community college to become a firefighter. There are other ways than joining the military if he wants to do that.
On a side note, I wouldn't trade my time in the good ol' USMC for anything. I'd do it again in a heartbeat.
Best of luck to your DS and his future.