Yes - he needs to give examples from the book. For example, Atticus Finch is happy/proud/lonely/intelligent/kind/whatever "we know this because Atticus "smiles/reads a lot of books/said this that and the other.
One thing to remember is that you can't really be wrong with what you think of his character - as long as it is backed up by the text.
I gave my DS my old notes on essay writing to help him. If you don't still have yours not being an obsessive hoarder who keeps old notes there are loads of online guidance sheets from secondary to masters levels.
We are similar personalities and prefer to read something and work it out rather than have someone standing over our shoulder. Handing him a sheet with quick explanation was the best solution to prevent an inevitable argument.
You've got it cracked wildthong. At this age it is all quite formulaic - see BBC bitesize. As long as he listens to the teacher and takes the concepts on board (I know, it's a big ask of teenagers) then he should be fine.
Work out 4-5 things to say about AF. Put them in a logical order. e.g. start domestic and spread out, demonstrating how he is in the wider world. How he is as: a parent (unconventional, preoccupied, vague but fundamentally good.) a lawyer (brilliant and genuinely fair) a member of the community (prepared to stick his neck out and become unpopular for the sake of truth and justice.)
A good way to do this is to quote what others think of him (judgemental?) and how he truly is (Scout's opinion, perhaps?)
How does he challenge the reader? How many of us would be as brave in his position? How many of us would risk our children's tranquillity and social standing to do as he does? etc.
So you can start with the small scale (how he is at home, as a parent) and then broaden it out further and further - at work, in the community, and finally, outside the realms of the book, to the reader, to our society.
Your DC must back up each statement he makes about AF with an example from the text. This can be a quotation or it can be a description of a scene - something AF does.
Make it roughly one paragraph per comment, add a brief intro and a good summing up, and there's the essay.
If he loves Eng Lit he can have more fun with it, and do a thesis antithesis essay - e.g. write 3 things that show how hopeless Finch a=is, and seem to be developing an argument against him as a heroic character, then turn the essay on its head and show how despite these weaknesses, he is a heroic man. That can be entertaining to read, and demonstrates independent thought. But maybe a bit advanced for a 14 yr old.
It's also good to try and make the intro a bit more exciting than just: ' in this essay I will explore...' It's fun to appear controversial at the outset. Maybe start with a couple of quotes that show Atticus in an unpopular light and build an intro that says despite his unpopularity among his community, AF's character wins through.