I agree with pp , you need to get him muzzle trained but first you need to get one that fits .
Your vet / pets at home should be able to help with that.
The training will take a while , it's the kind of thing that shouldn't be rushed at all.
Obviously all dogs are different but roughly .
1 , First few days leave the muzzle where your dog can see / sniff it, near his bed/ food etc.
2 , Hold it let him sniff it , take it with you when you go for walks.
3, By end of week 1, sit on the floor hold the muzzle facing your dog in the other had hold a high reward treat at the 'nose' end of the muzzle so your dog can smell it through the cage.
As soon as he puts his nose close to the open end praise and reward instantly.
4, Bit by bit encourage him to put his nose in a bit further, this can take a while , be patient.
Feed him through the cage to encourage him to keep still and not try to back out of it.
5 , Once he puts his nose straight in every time and keeps still rub gently around his ears where the strap goes, Then gently fasten it.
6 , Release it straight away to start with , then keep it on for a minute, 2 mins ,3 mins etc .
7, He is likely to try to use his paws / furniture to rub it off so make sure you have some fun things to do , favourite toy, go find DH / DC etc.
8, Only when he and you are really comfortable with putting it on and not trying to get it off should you go outside with with him wearing it.
Under NO circumstances should you ever force your dogs nose in to it
Don't let him chew / mouth it , it will only make it harder in the long run. It's a training aid , not a toy