Forget the holding of the muzzle, pressing his tongue, etc. That sort of action
hasn't a hope of being successful (well, you've already found that out
).
What your dog needs to learn is how to control the force of his bite (something he clearly has no clue about so far) and after that, that human skin is far too delicate to sustain more than the slightest pressure. Once he understands both those things - and ONLY after he understands those things - can biting/the use of his mouth be safely phased out.
The above is actually a lesson that he should have been taught from 8 weeks old, and would normally have you now at the point where the biting was being phased out altogether. Since he knows nothing of bite inhibition though, you're going to have to start at the 8 week old puppy stage and try to work your way up.
You need to think in training terms that your dog can understand : this is why holding his muzzle is pointless. What can the dog possibly understand from that? Actually, about a dozen different things (with "don't bite" being near the bottom of the list). It is about as unclear as it is possible to get.
Instead, you need to react to his biting in a way he can understand. The message is "OUCH!!!!!! - that hurt! Go away, I won't play with you if you hurt". Here, fortunately, the message and the way of getting it across to the biter is pretty much the same across all species. What does a human kid do when another kid bashes them on the head, or bites? Same thing as a puppy does when his littermate bites too hard. They shriek in pain, turn away from the one who hit/bit or otherwise hurt, limp off to lick their wounds and generally refuse to re-engage until after it stops hurting. THAT is the reaction you should be giving to your puppy when he bites.
This reaction is quite clear to the dog - he bit too hard, and nobody will play with him when he bites too hard. So he tries to be a bit gentler next time. Invariably, he'll bite too hard again and get the same reaction. But over time will learn to control his jaws better. This is the precise reason that nature gives young puppies such incredibly sharp teeth (much sharper than the adult dentition your pup now has) in the first place - so it will hurt when they bite, with even the slightest pressure. The dog then learns to control his jaws BEFORE he develops the strength and musculature to do damage with them.
The bite inhibition process thus involves you giving the dog the right feedback - that it HURTS when he bites. As he learns to be a little more gentle, you escalate the point at which you shriek in pain. That is, initially it is when he bites reasonably hard. Then when it is less pressure, then just a little pressure, and so on. Once you reach the point that the dog only ever uses his jaws incredibly gently, THEN it is time to phase out using his mouth altogether. You should NOT do it before - because if you do, then he will never understand how to be gentle or that human skin is fragile. And if he doesn't understand that, then some time later in life when he has cause to give a warning or reactionary nip (e.g. someone falls on him heavily, or inadvertantly slams some body part in a door), his nip is likely to be a very hard bite - since he has no idea of the power of his own jaws or how fragile humans are. Most especially for dogs that are ever going to be around children, I think you can probably see what a vital bit of education this is
Here is a short article that talks a little about teaching bite inhibition :
Teaching Bite Inhibition | Dog Star Daily
There used to be an excellent adaptation of Dr Ian Dunbar's method of training bite inhibition online, but unfortunately the site that had that is no longer around. That being the case, I would very strongly recommend that you get a copy of Dr Dunbar's booklet "After you get your puppy" (which is inexpensive) or consider some of the Sirius dog training videos that deal with this.