Your boyfriend sounds a lot like myself. If you traded the car projects out for art and crafting projects, you'd basically be describing me. I also didn't get the help I needed in school, and because I was having issues getting things like homework done, I was treated like a problem child despite good test grades. On top of issues with attention I have depression/anxiety. I've been told my attention is a symptom caused by the depression. I think the only thing you can do is encourage him to see a professional. Whether it is just ADD or something more, he should get a professional opinion/diagnosis so at the least he'd be able to better look into how to manage himself.

I've been actively working on myself for a long time, no longer with professional help because I cannot afford it, but I've greatly improved my hygiene, and, while it's difficult, I'm finding it in myself to actually work towards making my daydreams into a reality. My point being, I don't think he's hopeless but he absolutely has to be aware of himself and work on it. Not just say or promise he will, but show it through actions.