Duolingo ABC. Its free. Depending on where his reading is it might be too easy for him (good confidence boost if it is) but it has a library of readalong books that you unlock as you go or can just unlock the 'library'. It has a mix of letters, phonics, sight words which feed into the stories which you interact with in different ways.
Also the BBC has some good learning resources on their website. Worth checking out if you haven't.
What I did from the moment my daughter was born is read and talk to her. Don’t talk to them in baby talk or lots of slang as it can be confusing when they don’t find these words in actual texts(school books and such). We did a program called “The Million Word Gap”. Basically you talk to them as you would an adult. Using all the language you can. If they don’t understand a word explain it to them the best you can. My daughter was talking in full sentences by age two and no baby talk either. Now she is 9 and in Advanced placement she is reading at a middle school level and collects text books for “light reading”. Start out small wit your son and find books on things he’s into. Make it an adventure. My daughter, for a while, only wanted electronics until I found some books about her favorite things (animals, she wants to be a vet). Now she buys National Geographic dictionary type books to study everything she can about them. It started with a love of Marine Biology and evolved from that
It’s not easy but it doesn’t require drilling them for hours. It does take time, patience and lots of love and encouragement. I told my daughter that every book is a new adventure. So she goes adventuring everyday to far away exotic places. Hopefully you get the help you need! Best to you and your family.
Can't help much with anything except to suggest looking into JumpStart. My knowledge of them is going back a bit, but from what I remember all learning is done through games. I remember when I was really little my mum got a computer game to help with my typing. The company came up again while I was in school for therapeutic rec.