I'm certainly reading your advice with interest because my oldest boy is in the process of rewriting his first book to improve it, and has just sent me some pieces to read over.
In his first attempt (published via Amazon) I saw a few mistakes and was itching to correct them, lol. Apparently Amazon wouldn't let me. But now that he's withdrawn the book I can finally help him (somewhat).
I think that even sitting down and giving it a go as far as writing a book is concerned is a gutsy step. So many of us simply continue to dream or talk about that elusive "one day I will ... "
Absolutely! It's a massive step, a huge commitment and will probably take up way more time than he thought, even though he's done it all before.
An extra set of eyes will help enormously, but I always recommend a professional edit because an Editor worth his or her salt will improve the book beyond even the Author's expectations - if the Author allows it.
Please tell your son Good Luck from me!