There are multiples bites that can help you depending on what you call a "heavy" horse but you need to be careful because once he is used to one he will start to pull you again and you will need to find something even stronger.
I could advice you hackamore, gaucher bit, 4 ring gag, elevator, one with a toy to relax him or even try in resin (this is nicer, it won't be able to pull and be relax).
More important that the bite, you need to work for him to find a better balance, find light, don't give him a point where he can lean on.
For that, take and release, on one rein not both, try to relax him by "playing with your hands" not moving them but making some movements so that your contact is not too rough.
Work a lot on long reins so that he carries himself, find his own balance and ask from himself the contact.
Make a lot of transitions to stop him from pulling (when he starts go back to the lower gait).
Finally, you can make full neck extensions and work a lot on lateral movements to light him up. Also, don't forget to put impulsion, this is mostly why they pull down.
I hope i could help you :)
The mare I used to ride is quite strong so i went from a sanffle to a heavy big bit. That made her more strong.
I don t no your horse, but some strong horses needs to go on light soy bits to be confortable. I hope your question was asnwered