The heir] is, of course, a son, as Galatians 4:2 shows. A child] i.e. under age; a minor.
Differeth nothing] as respects the control of his destined possessions, though, in prospect, lord of all.
Related Commentaries of Galatians 4:1
Galatians 4:1
1 Now I say, That the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;