13 y/o, shoots, and actually sells some video along with making shorts for his YouTube channel.
He uses a point and shoot that shoots video now, but he needs a better camera.
His main concern at this point is audio. We'd like to get him something that has an external microphone.
I don't care if its primarily a video type or still type camera.
Ideally, I am looking for something used as to get most bang for the buck. Something like a really nice model from a few years ago. Bells and whistles are nice, but simple would suffice.
I only want spend 3-400 bucks.
Links appreciated.......
Thanks in advance.

You can get something like a Zoom H1 and sync the audio. You will get far better audio than buying a new camera

Your budget is too small.

A DSLR will get him better video.

For improved audio, regardless of the camera, you need an off-camera microphone.
There seem to be good mics that will mount onto the flash bracket.