He loves to lick me but lately he starts getting frustrated when i try and go into his mouth. Not sure why but i I don't wanna hurt him so i just give up after a minute.
Dogs generally never suck like a human. Get a bear if you want that.
If you see a dog sucking a dick entirely in his mouth the dog is either trained, forced or his suckling reflex that sometimes does not go away is take advantage of. All of these options are bad as they take away his consent. So just forget about this kind of oral sex with a dog. They do not do it.