Hi maryy16,
There is always room for improvement but it's great to see that things didn't escalate to extremes (you stopped replying)
Also the fact that he did finished the project and how it affected his mood (positively)
At one point, it looked like he was struggling with something, so I went over to help him out. At this point now he had gotten "the look" which I knew meant that there would be no more "teamwork." But he looked like he needed help, so I tried to help him out.
Him (Sarcastically): Did I ask for your help?
Me: No, but you looked like you needed help.
Him (Angrily): I don't need your help. You always get in the way. Don't do anything unless I tell you to.
Several things here. You thought he was struggling and decided to assist him. This could feel potentially invalidating to him ("Ok, so you don't think I can finish this DIY project on my own".
You saw his look of "no more teamwork" and knew what it meant but decided to keep trying to help him. I think the non verbal cues are warning signs to back off immediately. From what you wrote, this lead to his sarcasm, then you maintained your stance, finally he got angry and let you know exactly what he wants.
Make good note of it for future reference
So, a while later... .And I'm sure you all can see where this is going... .He's clearly needing some help again. So as to not "get in the way", I stand waiting to see if he's going to ask me to help him out. I actually feel dumb just standing there because he's clearly struggling.
Him (very angrily): Thanks. Because you didn't help me, I cut my leg ( which he did NOT). Why were you just standing there staring at me?
Very similar situation, reinforced, could potentially escalate.
You might consider doing the following. Next time tell him it looks like a great DIY project and that you will be in the other room doing other stuff. If he ever needs help, he can call you and you'll be glad to assist him. That's it. Don't stand close, don't suppose he is struggling (even if it's obvious), let him ask for help if he needs/wants it. Keep on with your own thing.
He finally did finish the project, without any help from me, and was in a good mood AFTER he finished.
This is awesome. Finishing projects feels like a great accomplishment to him. Many people start and never finish. It's a great achievement, something to celebrate and to keep doing more and more. From smaller projects to bigger ones.
Best of luck
mw