From what you describe, when asked for "more" forward, he simply falls forward faster while his hind end lags out behind. That is the easiest way for a horse to move. Takes more work for the hrose to engage and work his hind end under him. There are just tons and tons of threads on here about engagement and "collection" and such and you could do some scanning through to see if your situation hasn't already been covered. It's totally fine to ask again, of course.
I think sometimes when a hrose has been trained incorrectly in WP they learn to go this way, with the hind end just hanging out behind and the hrose "falling" onto the forehand as they move foreward. Initially, to get more engagement of the hind end, you may need to get him moving MORE foreward, and maybe it'd be faster, too, though you say you want him to slow down.
YOu might have to really move him out forward at first so he must engage his hind to push more. Any time a horse acceleates, he should be using more push from behind. So, do things that require acceleration, like going up hills, over cavaletti, some pretend "barrels" or just some work going up and down in gaits.
After the hrose is better at jumping into a proper accleration, then you would begin working on disallowing him to fall onto the forehand and moving his center of balance back to his pushing hind legs.
There's lot of people here who know the sequence of training for this better than I do. But for the first thing, if te horse has spent a lot of time being held to a slow gait, then I'd work on getting him to open up and Go!