Bit fit, and new bit ?s

Got a new bit for my boy. Its not knocking his teeth, he mouthed it for about 20 minutes before he settled down and finally started carrying it himself, then I took up a bit of slack.

He is sensitive to change and a big baby, so he hasnt been ridden with it yet but he wore it for 30 minutes the first day.

What do I need to do when I go back out to help him adjust to it more?

What sort of bit is it, and what was he in before?

Having asked that, I would say that bits aren't like girths that need tightening after a bit - it should fit at the right height in his mouth, which was either before or after you lifted/it. I'm guessing that if you took up 'slack', then until you did that it was hanging too low, and he was chomping at it with his teeth.

If it is in the right position in his mouth (put it on, open up his mouth and look where is is sitting), and assuming that it is not a very severe bit, then you shouldn't have to do anything else. You should just ride him

I was told that since he is still green to let it sit low, not knocking teeth, but just a little low so he learns to carry it with his tongue. He was mouthing it but not chewing it that I know of. Raised it just a bit after that. It's not touching any teeth in the front or back.

He was in a ring snaffle bit with twisted copper wire and jointed.
The new bit is an iron racing D bit with a single joint as well.

I bought a pamplet on bits and have been reading, he used to mouth the copper bit constantly, this is the first time I have seen him cary a bit well.

I just don't know where to go from here.

He probably mouthed the old bit due to the twist, not the copper. Mouthing the bit is not a bad thing. I think I would not put that bit any higher. If you can, loosen the throatlatch. It looks constricting.

Thanks! I could fit 4 fingers horizontally on the throat latch, I tried to find better pics but they are all wonky.

I don't ride with his bridle over his halter, but I didn't want to tie him with the bit since it was his first time.