Boomer can be found on the south side of Rae Rae's Pumpkin farm, in John's Region. You can get this marker early by talking to rescuees and listening to answering machine messages.
Boomer himself is caged up, and encircled by hostiles. There's also one in front of the truck, but that's as spread out as they get. Initially.
They're all pretty focused on fixing the truck or tormenting the dog, and if you're quick you pick them off 1-2-3. Start with the truck guy, as he's hidden from the others.
Knock down the rest. If you need to regroup, consider shinnying up the silo ladder, or dashing inside the house to either hide or set an ambush.
When you're clear, shoot off the chains on Boomer's cage to free him. This also starts his personal mission, 'Man's Best Friend,'
This is the easiest of loyalty mission- all you have to do is Pet Boomer. Follow your onscreen prompt to do so- you do have to pet his head, which needs to be facing you at the time.
This completes the mission, and unlocks Boomer as a Specialist companion.