I find it hard to  speed up his progress.

When I went all-out and spend 5 hours with him in the library, he would  read perhaps a few hundreds words. Most of them were either already familiar or uncommon. Out of those common words he needs to learn, he doesn’t get enough refresh (Ebbinghaus style) so not effective at all.

He also has a limited stamina. Beyond his  green zone, it’s very hard to get him to concentrate.

Endurance capacity increases with age.