I share your concerns Thistlelass. This seems different than the normal 6 year old boy obsessed with willies, farts and bums.
The lick my pee pee, lick the dogs bottom and watch adult movie aren’t the kind of sexualised comment I’d expect from a 6 year old.
It’s possible an older child has shown him pornography on his phone. It’s possible he’s being sexually exploited. Write down what you remember, what was happening when he first saud these things, who and how responses followed. Could you meet with his parents? If this was my 6 year old child or grandson I’d be worried.
If it was referred to a social work team they should investigate. Whether they would these days is another matter. Nspcc doesn’t investigate, it takes the details and sends them to the local authority sw team. But, nspcc has good, experienced staff who’d be happy to sit with you and talk this through