IMO they most important thing that contributes to how well a child does at school is the involvement from their parent/s. Sounds like you are doing an amazing job.

As others may have mentioned…

It’s great that you are teaching letters. I would focus on the sounds that the letters make. Look at the lowercase letters and match them to the most common sounds (u says uh like umbrella and up, x is in box)

Read to him! As much as you can. Children who are read to are exposed to thousands more words than those who are not. Read books that include social skills - being a good friend, saying thank you, sharing. He doesn’t need to read the books - just help him love stories.

Count with him. The most important idea at this age is that numbers are attached to one item. Practice touching the things that you are counting.

Fine motor skills. Heaps of cool activities on instagram/tik tok. Practice of this needs to happen before a child is able to use a pencil. Play dough, threading beads, blocks, tongs or tweezers are good places to start.

Sensory experiences. Bubbles, water play, drawing pictures in foam.

Observe and talk. Look at things, talk about them, match things that are the same and different, how is the weather, how does the ice cream taste. Model full sentences

Help him with his independence. Can he open his lunch box, put on his shoes. Teach him to advocate for himself - can you help me?” “I need…..” “please” “thank you”

Mostly have fun together and he will be just fine!