Teaching him from the first (number 0) St Joseph’s catechism is a good idea, and using the Catechism of Trent yourself so that you end up knowing enough more than he does to answer questions he may come up with is good, but do let him ask you questions rather than trying to give him explanations that are over his head.
Also, teach him other things, not just the catechism. Teach about the saints–children’s stories on his level. You can read to him these stories, children that age usually like that and it is good for them. (AND bible stories)
Also teach him prayers, the two of you pray them together.
Remind him of little things throughout the day. If something good happens, remind him to say thank you to God or to his guardian angel.
Remind him that God made things, like if you see a sunset, say "What a beautiful sunset God made for us!
If something bad happens, teach him to offer it up. I always use “All for Thee, dear Jesus, who has suffered so much for me.”
Most of all, teach him that God loves us and that Catholicism is not about following rules, but that there are rules to help us.
The most important thing to teach him is to be close to God, to Jesus, and to the communion of saints, in a friendly way.