I'm with JDI and SR. If he is refusing to move forward, he is either physically uncomfortable doing so, or confused about what you are asking. If you know how to get a green horse to move forward, then the former scenario is more likely. If you rule out all pain/discomfort, then he is probably confused, either not understanding your cue or not properly taught in the first place.
Assuming that you are asking correctly, if the horse is refusing to move forward there is probably a physical reason - something is making forward movement uncomfortable. Double and triple check saddle fit, ideally having some very experienced, preferably trained saddle fitting eyes take a look. The saddle tree may be ok, but, if we're talking an English saddle, the panels may need reflocked. Is the saddle pad appropriate? Thick enough to do its job, but not so thick as to affect saddle fit? Other things to look in to in terms of horse comfort are chiropractic issues or joint stiffness.
Once all physical pain has been ruled out, it's time to critically assess your riding. Horses 100% reflect what the rider is doing; be sure that your seat is allowing the horse to come forward, your hands are not blocking him, etc. Squeeze with your legs, give a verbal backup (a cluck or a "walk on", etc.), and if those don't get him moving back up the cues with a tap from a crop, dressage whip, or mecate popper, whatever your preference. Don't kick or spur, as that will simply lift the ribcage, stiffen him up, and make him dead-er to your leg.
As KV said, 2 years off is a good while. Take it slow. I recommend making sure that he has solid gas, brakes, and steering on the lunge and in hand. That'll refresh his memory if he was trained once, and will expose any gaps in his training that need fixing.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown