Below is a link to what Ralph Maltby wrote about fitting juniors. It’s the best guide you will find.
It is a guide and there can be a lot of variance in the size ans strength level of 7 year old boys. I have always used the charts as a starting point and then tried to dial it in using my own eyesight and judgement. Have him stand like he is addressing the ball, using a long ruler or yardstick or something that allows you to see what looks right for his height. Once you determine what looks right (imagine a mini version of you with an 8 iron in your hand), mark it and make the club that length. I have always fit juniors as simply smaller versions of us, wanting them to have the right fit for where they are at that moment in time. Gives them the best chance to develop good fundamentals. If the clubs are too long or too heavy, it will be more difficult for for them to swing and have success.
I know that’s way more info than you wanted, but hopefully it will help.