How can I get my kid to read more books instead of playing on his tablet all day?
Try taking your child on a weekly trip to the public library. Talk to him beforehand about the sort of stories or topics he likes to read about and help him find some that can be finished within a week. Better to take out too few than too many at first, so there is something to look forward to getting on the next trip. And let your child see you taking out a book for yourself that you'll also read during the week.
Make going to the library weekly a special sort of outing--stop for ice cream, or pizza, or lunch afterward. If you don't already have them, make a big deal about the two of you getting library cards.
Once reading becomes more of a habit, you can also allow the child to pick out a movie at the library to watch during the week as well, since that will also help to cut down on the time spent playing games on the tablet.