From a training standpoint, assuming the obvious medical and physical questions are addressed, I would reassess how you (and your trainer if they also ride the horse) ask for forward. And maybe more importantly, how you handle it when he protests going forward by humping up his back and whatever else might go on.
So to tackle the forward issue, it sounds like you need to get the aid re-established. Start on the ground, leading in hand. Carry a dressage whip so you can reach back toward his hip without changing your body position. Give a cluck and walk on briskly. If he doesn’t immediately march off with you, alongside you, give a firm tap with the whip and keep moving. Do not pull the lead rope, this is a hind end/response to aids exercise. Come back to a halt and repeat as many times as it takes for him to get tuned in enough that the second you step forward, he is right there with you.
Once you’ve got a feel for that, start the same exercise under saddle. From halt, give a quiet but clear aid form seat and leg to ask him to walk on. Not a crawl, a WALK. If he doesn’t immediately step off your leg into a forward walk, give a correction from either your leg or your whip/crop. There are some tricky elements to this though. First, you don’t want to whack the crap out of him, but the correction needs to be just that - a correction! It has to be firm enough that he shoots forward for a few steps so he really understands that you said something (your light aid) and you meant it. Second is that if he gives a BIG reaction to the correction (which is OK), that you don’t accidentally catch him in the mouth or clamp down on his sides or the saddle, because that would then be punishing/telling him to stop advancing. Third, and very important, is that you must quickly return to a halt and ask for the upward transition again using that same quiet aid you originally asked with. See if your correction was successful. If he doesn’t promptly respond to that light ask, repeat the exercise until he does. Once this is reliably working from Halt-Walk, start doing the same at a trot and canter.
If you are consistent with the aids, you follow through on correction when needed, and you always come back and ask with the quiet aid again, they usually sort this out very quickly. Also remember, forward does not mean fast speed, but it does mean fast response. Think of your horse as being forward in his brain, meaning he is paying attention and ready to react (appropriately) when you say it’s time to go.
But - all of that
is irrelevant if he is having a physical issue like ulcers or pain somewhere. So just make sure you do your due diligence and check all those boxes first. And remember, he’s young. His mind and body is still developing so your consistency now will make a big difference for the long term.