• Nioh 2
• Witchfire
• Devil Slayer Raksasi
• Curse of the Dead Gods
• Metal Mutation
• Cavity Busters
• Waves (free, but still)
• BlazBlue Entropy Effect
• 30XX
• Nova Drift
• Quantum Protocol
• Deep Rock Galactic
• Hellsinker
• Twin Ruin
• Devader
• Arboria
• Bloody Spell
• Aura of Worlds

Also, if he’s a bit of a tinkerer, he might be interested in trying shooters using gyro+flick-stick, which he probably didn’t have access to before. Witchfire, Deep Rock Galactic, and Deadlink can readily play that way once set up in Steam Input. Some games you only need to set up the gyro-to-mouse and flick-stick, whereas others (eg Vermintide 2) you have to map the entire controller manually.

If this added itch.io it’d be a fantastic filler for managing all the non-DRM off-Steam stuff.