Just go get books at the library and read to him. Work on letter sounds in a fun way and grab some bob books to practice reading but dont push is. Also be careful of is he actually reading or just memorizing. Both are great but if he is just memorizing words he knows that they look like vs being able to take a word and sound it out it will be a different approach. If he recognizes words then you try to get him to read sentences you may add stress and stop him wanting to learn to read instead of doing more. If he is sounding out words he doesn't know and actually reading words not just taking them from sight memory I'd focus more on just reading as much as you can to him and expand his vocabulary and it will make reading easier. And while doing that get some BOB books or similar and work them in so it isn'tsnt a chore but just fun reading time. Early reading is a fine line to walk because it can backfire. My daughter was reading board book by 3.5 by herself and while great we had to be careful with her pushing herself, getting frustrated, and 3 yr old emotions wrapped in an activity not the general norm forir a 3 yr old.

I think you’d be surprised to find what experts think about reading that early. I would recommend researching more on reading in early childhood. https://www.boredteachers.com/post/learning-to-read-too-early-might-be-counterproductive is just one of hundreds of articles.