I think the IO and lawyer is right and what you can do now is to keep your son's nonimmigrant status until the EB3 visa number becomes available.
You have reason not to worry about "age out" because your son did "seek to acquire" LPR before he reached 21 even if his previous I-1485 is deemed to have been abandomed due to the failure to get AP. It is an arguable issue and I think the stake is on your side.