Are you just sqeezing with your legs (both legs at the same time for a second, then release), or are you actively encouraging him to be forward with your seat as well?
A visualization that works well for me is to pretend that the horse's ribcage is like a big 55 gallon drum (bear with me here
). Your legs are going to keep the drum swinging from side to side. When his ribs seem to swing to the right within his walk stride, "catch" him with your right leg and gently "send" the drum back to your left leg. Basically, you're encouraging the natural motion of the walk to continue a little faster. The "sending" motions of the legs also help me to feel the walk with my seat as well, and from there influence the speed of the walk.
Some horses are just pokey, but he can learn to stride out and speed up a bit when you ask. You'll probably never change him into a voluntary speed demon, but barring some physical reason he can't speed up, you should have "gears" at each gait.