I can think of 2 options that'll let your son have full editing permsions (meaning he can add and edit extra family fields, as well as what ever else gets added that's restricted to founders)
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Down load the tree and give him the file.
Assuming you son also ends up using Family Echo, when he reuploads the tree it will say he's the founder and the edit history will be lost
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Go to your son on the tree, make an invite link, copy the invite link, remove the tree from your account, use the invite link to open the tree, and readd the tree to your account with Add to Account (top right of the screen between Save and My account), then just give you son your account.
This will keep the edit history show you still as the founder, and let your son have the extra permisions the founder has, even though he's not the founder.