Currently not much. If he is a toddler or very small child the best option is just have him share an Apple ID with another young child for the time being.
When older members age out leave the Family Sharing group, you can then give him his own Apple ID and add him to the Family sharing group when a slot opens up.
You can offer Apple feedback about it and they may choose to add support for a larger number of members but currently only 5 members plus the Family Organizer are supported.
click here ➜ Product Feedback - Apple