Do you have Sprint + Bullet Time bound to non-default keys on your or your brother's save file? It could be that when you updated MCM it reset the keybinds to their defaults, and IIRC any keybinds made in MCM are specific to a particular save file. In that case you'd be able to use things just fine while he wouldn't. I'd try having him rebind the sprint/bullet time keys on his file and seeing if they work when bound to other keys. I'd also double-check to make sure he has sprint/bullet time turned on in the Project Nevada options.
