His inclusion depends on the qualifying relationship, an unmarried child under 21, together with his own admissibility and an available visa. A delayed record is an evidence issue, not a change in relationship. Raise the delay with counsel now, because his timing sits inside the family instructions, not household preference.
Age and marital status matter to a pending case differently than to a granted one
Promptly review the legally relevant age and any applicable CSPA rules, without assuming the document delay protects the child. Later inclusion remains subject to eligibility at the relevant time; the US$15,000 fee and US$1,000,000 gift do not themselves overcome an age limit. Permanent residence already granted is not automatically cancelled by a later birthday or marriage.
The FAQ recommends initially including an eligible joining spouse and unmarried children under 21. An accompanying spouse or an unmarried child under twenty-one adds a further processing fee and a further gift of US$1 million for that person.