Can you tell what he likes and what his reactions are to his struggles? Like, is he enjoying this activity at all or is only frustrating him? How many different chords are you attempting to introduce at once? If you've tried all the chords you listed, are there any that are easier for him than the others? Answers to these questions can help guide your direction.
My suggestion is to try to determine which two chords are easiest for him and limit his repertoire for now to only those two chords. Do a search for two-chord songs and find some that will work. You might have to choose his one easiest chord and pair it with one of the others, even if it's not his second easiest, depending on whether the two easiest chords go together in any songs or not.
When choosing two-chord songs, figure out which ones he likes the best. Ask him his preferences or play them for him to see which he seems to prefer. If this process is a serious struggle for him, go very slowly in the beginning. His fingers getting sore at first may slow him down, too. Keep his practice sessions short until he builds some calluses on his fretting fingers. And only work on one or two songs for starters. When he starts making some progress, add in new songs slowly. When he's doing fairly well at playing the two chords, add a third chord and a new, 3-chord, song. And so forth.
Basically, start out really slowly. It will require lots of patience!
I'll be curious to learn how it goes.
Vicki