Please help me understand

Several days ago ADD H told me he wanted me to look at the state website with him because he was re-registering his car and was having trouble.  Last night I was working on something of my own when he asked me to come over and take a look with him. After standing around waiting for a few minutes (he was replying to emails and commenting on FB) I said "call me over when you are ready" and started to walk away.  He huffed and stopped but first had to find the email from the DMV, but where was it?  Can't find it either of his 2 email accounts.  I said go to the DMV website and forget the emails.  He finally gets into the website and shows me the form and he proceeded to fill it out as he had done so a "hundred" times previously and kept getting the same message to check the information entered.  He did this several times and was getting more agitated each time.  So, if you've ever helped someone with a problem, you have to ask questions to see where the situation stands and what other options need to be addressed.  I asked him did you try this? about that?...  He started raising his voice asking the usual, What am I an idiot, of course I did?  How stupid do you think I am?  I started to leave the room seeing no good coming from this interaction but he was standing in the way so I sat down and disengaged.   I told him I would not help him unless he lowered his voice and act civilly toward me.

The end result was that after he calmed down a little and let me actually control the computer, it took a minute to register the damn car.  He has vanity plates and was not choosing the correct category for the vehicle.  There were little bubbles to click on for more information for each line of information to be entered and where to find it on the current document.  I opened each one to verify the information was entered correctly while he squirmed because I wasn't going fast enough and I was wasting "his" time (I guess my time is ok to waste though).  I followed the instructions, found the incorrect information, corrected it and registered.  I said nothing just handed the computer back to him to pay the fee.  No thanks from him either but I've learned not to expect it.   I have nothing to say to him today, just moving on and acting like he isn't there.  

If you fill out a form a "hundred" times and keep getting the same message, how can you be so arrogant to expect the problem isn't some mistake you are making?  He is very smart but doesn't read or double check anything. He just expects he knows everything and everyone else is at fault.  Of course the state has a stupid website and how was he supposed to know that he was supposed to choose "other", it certainly isn't his fault!!  I'm thinking that's why they put those informational bubbles but dare not say so for fear of starting WWIII.   He has a very low threshold for frustration and reacts without thinking of the consequences.

I am very curious to know from someone with ADD or someone with insight why he feels he is entitled to act in such a roughshod manner, bulldozing over my feelings, and disrespectful when I am trying to give him the help HE asked for.