Understanding and Arabic: Try to tell him the story/lesson behing the surah he’s reading. Go slowly and gently, and don’t expect him to remember the first time you tell him. He will remember wrong and he most likely will jumble up the facts, which is why you have to go over it more than once(not in one sitting though!) Teach him some key words so he’s not just randomly reading without knowing. If he knows no Arabic take it slowly and do one word per surah; the more knowledge of Arabic he has the more you can teach him.
In terms of pre req, try to get him to understand first what the Quran is, and why we have it, and the rewards of reading it. It will mean more to him when he memorises it. This way, when you notice him slacking, you can simply remind him of his potential rewards and it will hopefully make him more eager again. Teach him the Sunan of recitation as well.
Teaching method: I would suggest full time Islamic schooling if possible, because that means he will be more well rounded Islamically. Do make sure he learns Arabic if he doesn’t, because it is imperative for understanding Quraan, because I can tell you from my personal experience, there is nothing sweeter than reading Quran in Arabic and actually understanding it. I’m still not as fluent as I’d like to be, but I can’t imagine what it would be like to not know the Arabic I need. The younger he is, the easier it is for him to learn. Arabic is not an easy language, but learning it at 3 is better than at 6, which is better that at 10, which is better than at 20 and so on.
Regardless of whether you put him in an Islamic school or moque/madrasa, make sure you get at least 20 minutes a day(not necessarily consecutive-remember, quality not quantity) to check they’ve memorised correctly.
Memorisation: It’s great that he memorises quickly-TAKE ADVANTAGE OF THAT. What happens if you put a child in front of a TV for a few days? He memorises all the songs and all the adverts-same tune, same pauses, same pitch. When you see the number of words, you find he memorised two or three Surahs worth of words. Which brings me to my point. Do not underestimate a child’s ability to learn from listening. Whatever Surah he’s learning, put it on repeat where he can hear it. It’s okay if his focus is not 100% on the Quraan, he will still listen to it. Encourage him to recite with the qaari’ as well. He will learn the tajweed simply by listening, though make sure to change up the qaari’ sometimes, so he doesn’t memorise it like a song.
Incorporating Quraan in daily life: Make sure you take lessons from the Quraan and apply them to your daily life, big or small. He doesn’t have to have memorised it, just make sure you read him the ayah, tell him what it means and explain the story behind it + the lesson learned.
Please make the most of your child’s clear mind and fill it with what’s good. The more he learns now, the better it will be for him later. Children are fast learners, don’t underestimate their ability to learn. I regret not taking more of an interest in Quraan and Islamic studies as a child, which now makes everything harder. I am now making sure the same doesn’t happen to my sister(5) and am making sure I can teach her all I can. Consider it sadaqah jaariyah. It is the best gift you can give to your child and no one can take it away from them!