Okay, the other poster answered about most of the basic stuff that I saw too (sorry I can't remember your name!).
You said he's a bit heavy on the forehand, so you might already know this, but I thought I'd through it in. He needs to learn that the reins aren't for leaning on. It'll help when you start looking up more. Roll your shoulders back, elbows towards your waist, nice, allowing rein contact, eyes up.
When he leans on the bit, ask for a little forward with your legs, and keep your hands allowing, but try not to let him go faster by squeezing them a little bit. Also, maybe try some half-halts, to really pick him up and get him using his haunches. Another thing you could try is just from time to time at the trot, drop the reins so you're holding the buckle and he has nothing to lean on, and make him keep trotting. He'll have to balance himself without leaning on the bit to do this. That doesn't work on all horses, but it does help with some in my experience.