Epic free games change weekly on Thursday at 11am EST unless it's during a sale when they could change daily.
https://store.epicgames.com/en-US/free-games
If you have Amazon Prime, games are given away that activate through Amazon, Epic, and GOG. You keep these games even if your Prime subscription ends. Check at least monthly, but new ones can be added weekly, but it's not a set schedule.
https://gaming.amazon.com/home
Keep an eye on the Monthly Choice, as well as normal Humble Bundles. GOG is also starting to do bundles, and Green Man Gaming as well. Fanatical also has crazy pick and choose bundles that occasionally have recent games down to $7.50 or less that haven't dropped before $15-20 elsewhere. Also check current and historic prices, and setup price watches to save month. I prefer using
https://gg.deals/
Easiest first time emulation is RetroArch installed directly through Steam. It has support for Remote Play Together so your son could play multiplayer retro games with friends easily. The remote player is only streaming the game, and the client can capture controls so there aren't weird networking desyncs.
Mods depend on the game. If the game has Steam Workshop support, it's just selecting the mod within Steam just like you would a DLC for a game. Otherwise there might be a mod manager for the game. Or it could be manually configuring things.