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 ******, don't let him get away with being a lazy sod!
He needs to be moving, WITH PURPOSE, EVERYTIME you use your leg. If you ask him to walk, it's not good enough for him to just move off the leg and plod along. You want him to move off your leg, immediately, like he's walking to his feed bucket!
When a horse has impulsion--immediate (but calm) push from behind--the back legs WILL be engaged.
Just moving isn't good enough. Having to push him along, isn't good enough. Having him take a few walking steps before you ask him to trot isn't good enough.
Kep your legs on him. I agree with Kayty. Lots of transitions. Trot to lope, lope to trot. Beat him with the crop if he doesn't respond. Well, abuse is bad but there ain't no harm in giving him a good whack to get him moving. I had the beat Jester's butt multiple times to get him moving. Lazy rat he was, but he got better after awhile ^^
Alright well I can get him to move at the posting trot but its when we do a sitting trot he just gives up and drags his behind. So is there anything I can do with the sitting trot or do I just worry about the posting trot?
Worry about the posting trot. Most likely, when you sit you inhibit his movement (by holding with your legs, not sitting quietly, holding on the reins, etc.), and he's not giving his back yet to make sitting easy for you.
Work at the posting trot, get him moving. (DON'T think this means 'rush'. Just purposeful.) When you come down from a canter to a trot, don't just let him 'trickle' into a trot and plod along. You want a nice moving canter directly into a nice moving trot/walk/ etc.