Velocity averages should be based on size as well as age.
From this information, he might be throwing faster than the average kid his size. You can only get so much out of a 79 lb body. There's always going to be things you can do to add velocity, but keep in mind that if all things are equal, he will not be throwing as fast as another 11 year old that is 140 lbs.
Just last week I had two 11yo pitchers out for a bullpen. Both really good athletes though one was about 70 lbs and the other was last measure at 150 lbs. Knowing the physical limits for the smaller pitcher, we didn't emphasize an overpowering fastball. Instead, we worked on movement pitches and locating the ball. Think Greg Maddux.
For the larger pitcher we worked on his fastball velocity and change ups while keeping it out of the sweet spots for hitters. Think Brian Wilson before tommy john. Both pitchers can be equally effective in a game, we just play to their strengths.