If there is a sudden change it is normally something that has changed (rather than just the nature of the pony), have a really hard think back about what changed a few months ago, did you change feeds? put him in a new field? get a new bridle/saddle/numnah? etc., if there is nothing you can think of so that you can switch back to see if the problem disappears I would get tack, back and teeth checked and possibly a full vet MOT to eliminate any pain issues.
If you get the all clear from all of those perhaps try not holding him back all the time, but instead turn him tightly every time he tries to rush forwards, this could take months of repetition in the school and you will have to stop doing all the fun things while he learns not to rush off, if you go back to holding him just once it would undo all the good work. First do it in halt, if he tries to walk make a tight 90 degree turn (turn on the forehand type tight) and RELEASE until you can mount and sit there without holding him back, only once you can do this for at least a minute and wriggle about in the saddle a bit without holding on should you try doing the same in walk. Once in walk, repeat with a slightly less tight turn but still 90 degrees then release, until he can walk several laps of the school/field without trying to rush. Initially you might do a turn, 1 step, a turn, 1 step and so on, round and round, he will get bored of that and gradually give you more steps nicely in between. Once walk is 100% perfect and he does that same perfect walk every time you get on without having to remind him, only then progress to trot.