Personally, I am happy you are keeping him home. In my opinion you are already doing the best thing for him. Choose what you like out of the following or the other ideas you are receiving, and he will be fine when he starts school.
You could probably just include learning his letters and numbers in your daily activities. Point out letters as you are driving, on the STOP sign for example, and when you read to him. Have him try to form his body into letter shapes. With my first child, we did a lot of letter and sound recognition in the bathtub with foam letters. Use sidewalk chalk, pudding, magnetic doodle boards. Count steps when you are walking. Count blocks or toys when you are putting them away. Have him help you set the table and count the flatware and dishes. Do simple addition, 1 bowl plus 1 bowl. You may already be doing things like that.
Hope some of this helps,