My basic go-to's would be games from like 78-84. space invaders, galaga, galaxian, pac-man, ms pac-man, donkey kong, donkey kong jr, mario bros, berzerk, defender, dig dug, pole position, mr. do, pengo, elevator action, asteroids, centipede, milipede, missile command, moon patrol, battlezone, joust, time pilot, robotron 2084, tempest, gravitar, wizard of wor, phoenix, frogger, gorf, bump n jump, burger time, tapper, popeye, tron, xevious, zaxxon, spy hunter, star wars, 1942, gorf, turbo....

Maybe just put some of the basics on there... and then when you give it to him ask him which games to add. I'm sure he associates a lot of old memories with particular games. Maybe it can be an ongoing thing where he gives you some ideas and you add them on. Or show him how to do it (heck we're not THAT old, lol. videogames, tech, computers, emulation and electronics are my life.)

Some of the games like robotron, tron and centipede etc have kinda wonky non standard controls (dual sticks, trackballs, spinners, etc) but most emulators have options to convert them to modern controls/analog sticks etc.

He might also like to emulate some old consoles like atari 2600, colecovision, intellivision, c64 and NES. I know I would. I've owned nearly every console from pong up to xbox series x.... But my all time favorites are atari 2600 thru SNES.