Buying games for my kids, without me "attaching" a credit card
You can buy an application in the website version of the Play Store to remotely install application in the tablet. This way, you'll be entering your credit card details in a computer you can trust.

In the tablet itself, if the child goes to Play Store to purchase a paid application, the last 4 digits of your credit card numbers will be displayed on the device. When you use the default setup (which is to ask to reenter password for every purchase), he will not be able to make any purchase unless he knows the Google Account's password.

If him knowing your last 4 digits of your credit card number is a worry for you, you can detach the credit card after every purchase. I think this is probably unnecessary, if he wants to know your credit card number, it will be a lot easier for him to just snap a picture of your credit card out of your wallet if you live together.

I recommend using a dedicated account for app purchase rather than using your own personal account (you probably don't want him reading your emails) or your child account (he probably should know the password to his own account). You can attach multiple Google Accounts to a single device.

Another possibility is to just tell him to buy the gift card himself. He wants the game and he has the money, he can buy the gift card himself. This is assuming he's mature enough to go to a supermarket/store alone.