Assuming he’s playing on a 1080p monitor then CSGO and DOTA2 will be ok. You won’t see super high FPS but you can run the game well enough to play and have fun. However, GTAV and PUBG will need some settings turned down into the medium to low range to see decent FPS. PUBG is poorly optimized even for higher end hardware. He could still play them and have a good experience but I would recommend picking up an i5 at some point.
The 660 and i3 combo is ok for indie games and older games. Anything current or demanding will struggle.