The easier option would be a dedicated recorder or control surface with a one button per function interface, maybe even DAW with a proper control surface and some help from a screen reader or accessibility voice prompts.

Old keyboards and synths that you don't need too menu dive into and are not studded with with multi-function rotary encoders would work best if he wants synth sounds, along with a braille tape printer to label things.

Making music isn't about a stack of synths and plugins, a stick banged on the floor, a guitar, a keyboard will do just as well coupled with an old cassette deck.

A little help from friends is always a good thing and makes it a lot more fun for everyone.

There is never any shame in asking for help. I wish more people would realize that, it would greatly improve their lives.

EdIT

Dr. DuckDuckGo has some info as well:
https://blog.prototypr.io/audio-production-for-visually-impaired-musicians-aff825394136

 

You misused the word soundproofing as that means making the room leak proof, the sound not being able to leave the room which would incur many thousands in cost of construction and possibly deconstruction. Treating the room is probably what you meant to say.