Who did his teeth and what tool did they use? There's a big range of expertise and effectiveness in teeth floating. Used to be that you really wanted an equine dentist; now, I think my recommendation would be for an experienced vet that uses the new power equipment. I also now prefer sedation because I don’t want the vet or dentist to stop floating or avoid an area because the horse is acting up.
You can experiment with a variety of different bits to see if there's something that makes him happier. Since he's going in a french link snaffle now, try a single joint snaffle or a mullen mouth. I might be tempted to try a hackamore, bitless bridle or sidepull to see the difference. Really, it just depends how patient you are and how much tack you have available to experiment with. I'd also consider trying a Myler or happy mouth bit. I would not ride him in a gag, (side note: I would never recommend riding a gag with a single rein - if you're going to use one, have a rein attached directly to the bit ring, and carry the gag rein like a curb rein. That way you can control how much leverage you apply) and if you use the flash, I would keep it buckled fairly loosely - if you crank it down tight to inhibit the behavior, you might also have it so tight you inhibit his mouthing and chewing the bit as you want him to. Coercion, or coercive tack, is not going to solve the problem and may make it worse.
From your description of it being worse in transitions, the fact that he had been working quietly with the bit previously, and the that you've had his teeth done recently, my suspicions are that it's either a symptom of pain/discomfort in his back or hind end or that it's behavioral and needs to be worked through in training.
If changes in bitting don't change the behavior, have a vet or chiropractor look at his back and hind end very carefully. Also check saddle fit. If you can't find a physical cause and you can't find a bitting solution, then you have to treat it as a training problem. A good diagnostic would be to lunge him in side reins and she if the behavior stays the same. You could also give him a couple of grams of bute, 1 the evening before and on the morning of your ride and see if that effects the behavior. If it does, that's a pretty clear indicator that it's pain related.
If you've eliminated pain and discomfort and none of your bit experiments work, you're left with riding him through it.
If he roots or tugs, do not let him root rein out of your fingers. I understand your trying not to play tug of war (always wise with on OTTB) but a better technique is to drop your knuckles on his neck while he roots and tugs so he's not pulling directly on you; pick your hands back when he stops fussing.
I would also not take my leg off in downward transitions - that's allowing the horse to train you. I would just continue to ride a lot of transitions and make sure I was riding assertively *forward* in all of them - being tentative with your aids will give him time to fuss. I would ignore, or pretend to ignore, the behavior and concentrate on forward and straight. You can also experiment with asking for the transitions inside a circle or other figure; seeing if either the distraction or having to maintain the bend through the transition helps.
Sorry - I feel like I wrote a book. Frustrating problem! Good luck and post back and let us known how it goes.