I have an 8 year old boy as well who has a similar problem with low tolerance.  He has in the past reacted the same way by hitting and will do so from time to time.  I try to remind myself that they are children and need constant promtping-  Especially boys.  In my reading of articles, books and seeking advice from experts, boys are not emotional stable as girls as well as verbally expressive and therefore will act out physically.  In my experience, I have told me son, which I'm sure you have done so it to use his words instead of his hands and tell the sibling how they feel (obviously staying away from name calling).  I will also do some role playing with that child.  In doing so, we replay the scenario again and he is then feeling more confident in handling the situation. 
Remember it takes a lot of practice and several mistakes and much praise. 

Good luck!