Does he react immediately when you put your leg on? If he doesn't walk off straight away, that's your problem, he's just ignorant to your leg.
I'd carry a dressage whip, and when you ask him to move off with leg and he doesn't react immediately, give him a tap behind your leg to wake him up. It won't take long until he realises it's much easier to walk off with the leg than wait for a whip.
Once you've go your go button down pat, you want to do a million transitions. Get him trotting, and if he's not active, put your leg on, if he doesn't go, give him a flick behind your leg. Get him hooning. Doesn't matter where his head is, you just want him to GO from the leg.
When he reacts to the leg in all paces, you can start fine tuning the forward movement, but for now, you want him to leap away from the leg. Little bugger, don't let him get away with being a lazy sod!