Since you don't say to what degree he has either affliction, it is hard to offer advice on this subject. Autism can cause him to be very difficult to potty train, as can ADHD. I suspect that the ADHD is your biggest obsticle, though.
Here are a couple things that I have had luck with...
Try using Cheerios or Fruit Loops as targets in the toilet for him. Make a really big deal about him hitting the target. Tell him how fun target practice is. If you use Fruit Loops, then you can also help him learn his colors with this- have him just shoot the green ones one time, then the next time switch to blue... You get the idea?
Another thing that worked with my older brother was that he got to pee in a jar. I know, it sounds trivial and gross, but he thought it was really cool to pee in that jar! And, after he was done, mom took it into the bathroom and showed him that she was pouring it in the toilet. Then, she started telling him that he could just skip the jar and go straight to the toilet. He did.
Another thing to keep in mind about potty training a kid is that no matter how hard YOU try, if they arent trying all of your efforts go wasted. Are you starting TOO EARLY? My daughter is just now potty trained- and she is a little over 3.
Good luck!