Put him in a rope halter, apply slight pressure and tap him on the bum with a lunge whip lightly (but make sure he can feel it). When he moves forward make sure to release pressure on the halter and move with him. Do this all standing at his shoulder so he knows he can be lead even if you're not in front of him. Hope this helps.
My horse still does this and hes 27!! I make sure tho that when i ask him to move and he does that i reward him so that he makes the connection that if he moves he gets a treat 😁😁
I used to have the same issue with my young pony. I usually don't like treats to entice and encourage behavior because horses tend to rely on that and can become too distracted by the the treat. In this case it was the only way to begin to get him to move forward. As soon as he even seemed to lean in the direction I asked him to I would reward him with a treat and lots of pats/scratchies and a vocal cue. I soon started to wean him off the treats and reward him with just pats and vocal commands, I also asked for larger movements. (A step, a few steps etc.) hope this helps, good luck with training!