Helping Your Child Develop Good Credit

It's pretty simple. Add his name as an authorized user of your credit card. This will help him build up his credit score and will make it easier for him to get loans, including mortgage loans, in the future.

New credit card rules make it harder for those under age 21 to get credit cards.

The Los Angeles Times says:

Adding a child as an authorized user to a card is a low-risk way to build his credit since you don't have to give him the card or access to the account. Instead, your history with the card is simply added to his credit reports, assuming your credit card issuer agrees (call and make sure first; some issuers report authorized-user information only for spouses). All versions of the leading FICO credit scoring formula factor authorized-user information into their scores, although the latest iteration — FICO 08 — limits how many authorized-user accounts are included.

When you decide to do this is up to you. A longer credit history is generally better, but you should add him only when and if you're comfortable doing so.