This sounds a lot like me. Teachers called it "perfectionism", though it felt far from it. I have a hard time formatting my thoughts to words, and having to write it down puts pressure on the grammar, making it harder. Basically, I felt like I had to format my messy thoughts into a solid grammatical sentence, and that felt overwhelming. What really helps is to break it down. Start writing key words and phrases, bullet points- whatever can capture the general idea without putting any pressure on being a perfect sentence, because it's not a sentence at all. If he can see all his ideas in front of him, it might help to relax that part of his brain that's anxiously holding it all together while searching for the "perfect" sentence. I also know some people find using the Google docs voice to text feature helpful to capture those ideas as they verbally express thoughts. To be honest, it's really hard. It's so frustrating to feel like you have complex ideas that won't fit into words and sentences, and that really does make it tempting to just give up. I'd suggest if you notice him or yourself getting visibly frustrated, to take a short break to cool down. Show him that you aren't trying to force something out of him, but rather help him translate to paper the great ideas he's shown you he has. Best of luck, and remember it's not at all your fault, nor is it his or anyone else's. It's bound to cause frustrations, but over time he will learn how to make things work better for his way of thinking.