I'll echo that you've probably just grazed a tooth. No need to panic, its a common occurrence when doing 'oral and handiwork'. Their penises are essentially bare flesh, mucous membrane with no outer epidermal layer, similar to our gum (oral) tissue. That tissue abrades very easily, eg. you'll sometimes see crimson on your hands just from holding behind a knot for a long time & giving oral up front, particularly if the dog is inexperienced (virgin) and the skin back there hasn't had ample opportunity to toughen, or if your hands are rough/calloused. This is often exacerbated by males thrusting and us trying to catch that early action in our mouths, as most males make this impossible to do carefully, so the tooth-graze probability here is high.
Perhaps a better technique is to simply lay under him and hold that knot snug in a well-lubed, warm-water-washed or even soak-softened hand (think: long shower/bath pruning). Liberal amounts of mineral oil work great for all-purpose lube. Let him thrust wildly to his heart's content just holding his knot for him during the thrusting, while you're under him to catch a good wet messy facial. Be creative, change it up now and again by catching his load (first and second fraction) in a glass to savor afterwards, or let him shoot it in your underwear to carry against you intimately throughout the day. When he's finished thrusting, you prop up on some pillows or go into a sit behind him when he turns, finishing the longhaul tie duration in your mouth once he's gone still. Pro tip: keep your head stationary, seal your lips around him, and use only internal tongue movements or repetitive suction instead of stroking/bobbing up and down on his shaft like we would with a human man. Why? Because squeezing versus stroking mimics normal mating: a bitch's vagina wouldn't move up & down the shaft in this way once they're tied; when he's done thrusting, he has no more need of this bobbing/stroking movement anymore. Though admittedly some males might come around to like the bobbing, I've found most seem to perceive it as distracting during the tie. When in Rome, do as the Romans. They tend to respond much better to 'stationary' oral with knot stimulation. Or more particularly, a constant tugging sensation supplied behind the knot over anything in the shaft stroking realm. I suspect they have similar motivations for dragging us around the room when anally tied -- they like to play a different kind of tug-o-war with us here, lol. And hey, when your head stays stationary, you're less likely to graze a tooth! Although I will posit that its fun (stimulating) to wake him up now and again during that long tie, with a firm tongue-press to his tip, similar to how they do themselves when licking their sheathes, or how equines belly slap for that pang of pleasurable resistance. Hell, we feel it too when we twitch, squeeze, or press into our own hands. This from a lifetime of anthropologically observing and mental-noting animal pleasures.
Anyway. Fortunately--as you've noticed--the blood supply is rich in that area, so it clots & heals rapidly. In a day or so, all's well and he's ready to go again. The tissue behind the knot will toughen up some too, benefiting from practice. It ought to stop leaving crimson on the hands after the first few encounters, milestoning his progress from virgin to experienced stud.
If you're determined to offer him mouth-hugging all the way through the act, including his early wild thrusting, you might try wearing a nightguard or sports mouthpiece ("mouthguard") over your teeth, making it safer for him. The nightguards are a bit more expensive but they're boil-and-bite, so they custom mold to your own teeth. "Richie! Put your bit in!" ? (From Dusk Til Dawn anyone? No? I'm the only one? Sigh).
