Hi, sweet of you to help your son with the game! (I am a grandmother of two MineCrafters, but their Dad does their tech stuff).
From what I can tell, not many mods have updated to be compatible with the newest update. Your son can still run the older MineCraft though. When you start up, there should be a window, to the left of the big green "PLAY" button, to choose what version of the game you want. You can choose the version he played with mods, and just keep playing his world(s) on that version.
Hope this helps!