teaching a kid to ride a bike

this time i'm not the frustrated one .but my youngest nephew is .hes a great kid but recently hes been flipping switches .going from happy to mad as hell in a split second .so trying to teach him to ride a bike is to say the least is interesting .his older brother has managed to learn to ride rather quickly and is doing well .hes a little scared of falling still so hes not wanting to go to fast yet .but i know he'll be tearing it up before to long .but back to his younger brother .i'm running along side him holding the seat to keep him steady .and when we're going at a good speed and i feel him balanced i let go .he rode for probably 20-30 feet doing great then he hiccuped of something and fell .he went from being happy as hell to mad as hell in a split second yelling "why did you let go you stupid aaron?" i laughed which i'm sure he didn't like but oh well .so he finally calmed down and we got him some knee and elbow pads (he already had on his helmet) and we tried it again .i'm running along side him still holding his seat and he steadies himself .so i let go still running along side him ready to grab him if i need to .i told him i had let go which probably wasn't the best thing cause he looked back to see if i was telling the truth and he became unstable and started to fall so i grabbed his seat and handle bars and managed to stop him from falling only to almost make myself fall .but after that he was happy and we turned around and were going to try again .we were doing good and i'm still at his side and i let go and for reasons only known to him he slams on the brakes i run past him and he falls .and of coarse gets mad at me