It's nice to hear that your son is doing much better after major surgery last year. At age 14/15, I have to assume that your son is an incoming freshman? If so, I wouldn't worry about his 60 yard time yet! He'll be going thru the typical maturation process over the next couple of years, and his 5'3" 120LB frame will change with it.
I'd do as gonefishin has advised, and work on some speed drills with him. I see he plays a number of positions right now. If he ends up as a corner infielder/outfielder, speed is a little less important. However, if he wants to stay in the middle of the field (SS/2B/CF), speed and foot quickness etc... is very important. If you want to gauge where your son should be at in the next two years or so, log onto the Perfectgame website and check out recent showcases. You'll get a good idea where kids around the country are at in the 60. Best of Luck!