Great choice on the cornet. Yamaha makes very high quality instruments.
I would suggest he meet with a physical therapist and get their advice about the best way to support the instrument. A consideration would be a saxophone neck strap, attached to the cornet so he can let it go unsupported for changing mutes, turning pages, etc. and still not have to set it down. We have a french horn player in pops band that uses one when going from his seat to his case, so he can use his canes. Cheap, easy to find, and less intrusive than a stand alone prop.