Been there, still there at times. DS is now 7, it's getting much better than it was at 5 or 6. We try to talk about the situation later, when things are calmer... tying in that it is normal to make mistakes, using examples from our own growing-up or even grownup lives, sharing something that he could not do at one point but later learned, etc.
It has also been helpful to share stories about how famous people tried, failed, tried, failed before they succeeded. We talk about failure as part of growing and learning. With humor whenever possible!
Since part of it is you being "better" than him, it might be that he wants you to watch or just be near him more than he wants you to do the same thing. Totally a guess, but it might remove one of the points of frustration.
