“When he saw the food/pizza, he started drooling at his mouth.”
Is this sentence OK?

It would be ‘from his mouth’ but in any case that is a redundancy. He cannot ‘drool’ from anywhere else.
‘When he saw the food, he started drooling.’

What is the colloquial expression for this sentence - ‘When he saw the food, he started drooling.’?

That is fairly colloquial. ‘He started salivating’ would be more formal.