loose brazeon
 

downtube water bottle cage braze is loose(aluminum frame). how can I fix it?

Your local bike shop has a tool for that repair.

Yeah, they aren't brazed on aluminum frames, chances are that it's something along the lines of this.

I have one of those that is configured like a pop-rivet gun. Shoot me a PM and we can discuss it.

It's common name a Riv Nut, they can be further expanded by putting a thick flatwasher and a second one,
need not be as wide , under the bolt, grease the threads, and tighten the bolt more ..

tightening the backside against its shoulder is how it was expanded , barrel like, into the drilled hole in the 1st place.