Good, I'm glad it helped!
Report writing: with unschooling, you can discuss opportunities and what you observe that your son has learned or is learning.
So you'd list available resources such as internet access, books, art supplies, a chemistry set, a library, local museums which he could go to if he wanted (irrespective of whether he's actually done that yet - just as schools are allowed to brag about having a pottery wheel or choir or football team even if a specific child isn't currently choosing to use those facilities), any knowledgeable friends or family or yourself who'd be willing to teach him about maths or local history or politics or sports coaching.
What he has learned: think hard and be observant about what he's learning. You can start making a rough list now. Even if the LA never get in touch, that will reassure you that he's making progress!
Does he get the bus to go see his friends? Then he's learning independence, how to read a bus timetable, confidence in interacting with the bus driver, geography. Maybe you've talked to him about his personal safety and whether there are parts of town which he should avoid at night (and why), discussed drug use with him, and told him what to do if he loses his bus card.
You know your son is socialising with you and his friends, and you may have seen him developing skills through that such as negotiation, tact, empathy, and acceptance.
You probably have conversations with him on a wide range of topics, from which he'll be learning. Yesterday I told my teen about a friend of mine who'd recently been suckered into accepting a job abroad. When he arrived, it turned out to be completely different from what he'd been told: dangerous and exploitative. He had the resources and sense to turn around and come straight home within a week, despite his shame at having been tricked. He'd resigned his previous job and lost £3000, but at least he was okay. We talked about what he could have done if he hadn't had money for airfare to come home. Who could he go to who might help him? I mentioned that some people are more isolated and vulnerable and perhaps have their passports taken from them. We discussed human trafficking and modern slavery in this country and elsewhere. The subject is of particular interest to me because my daughter has a learning disability and is too trusting, so I want her to be aware of how to avoid and escape from these situations.
If your son gets paid for the gardening work, then he has experience of running a small business, being organised, negotiating rates, appropriate workplace behaviour (what happens if you don't turn up when you said you would? what if they ask you to do extra work for free? can you help yourself to food from their fridge? what do you do if you're scared of their dog?) and managing money. If it's voluntary, then he's learning about altruism and community engagement. Obviously he's learning a useful skill and becoming more employable.
You can talk about progress he's made since leaving school with his mental health and self-esteem.
Education Otherwise has a template and a report-checking service, where knowledgeable people will look at a draft of your report and advise on whether you've gone into enough detail and whether there's anything missing which the LA is likely to want to see. That service is free to their members and available to non-members in return for a small donation. See the top of the page here: www.educationotherwise.org/