I don't think drugs are the best solutions,let it come naturally from the kid that he's willing to read.
First, kids have their favourite and worst subjects.Take sometime and realise which kind of subjects your kids enjoy to read and the one's de doesn't.Once you realise this you'll be able to understand the reason why he's not concentrating.Some solutions.
1.)Advice your kid that whenever he's doing some personal studies, to jot down the important things so that he'll go through after done with reading.
2.)Once he's done reading, he should take some minutes like 15 minutes to reflect on what he's been reading.
3.)When he's reading he should avoid destruction like phone and other stuff and concentratein the books for only a short time like 15minutes.
4.)Finally, when reading should always have some cold water aside and maybe at times have some refreshing chewing gum.Science believe that this boosts concentration.