My sister used to have a drinking problem and instead of her boys being put into the "system" since I was 17 and have my own place for over a year the judge upon my request gave them to me 8 and 10 year old boys.
I am sure if you introduce yourseld to the principal and teachers they will understand 100%. Teachers would have a better understanding of the situation with the boy and ways to help. Principals I have learned in the past are not so great as that as the youngest was dyslexic and nothing was done about it til I got them. SO I fully understand what you are going through. When mom came back after a year I still had them at night like you as well until I left for college at 19 making sure she'd take good care of them and they were ok having mom back. I also had found them a sitter I trusted too.
But talk to the teachers. I know they weill suggest reading to him, flash cards, and board games that require reading. The board games are easy to make right at home. If you need help on that let me know and I can help. I went to college for that reason but married an airforce man (well he was in college too then) and moved away and just never in one place long enough to finish.
Also find out what he likes. It is very important to play up his intrests when working with reading as it helps them want to read. See if you can take him to find books at what his lvl should be and if it is really bad a lvl below or so to start with and have him read to you b4 bed or while you are cooking or whatever if you are busy. It don't have to be long periods of time even 10 or 15 mins a day will help alot.
Bren