Abi you said it 'sex pest'. You called it. Nailed it, as it were 
That is exactly what it is
Your body is your own. You give consent around access.
Its a common abusers behaviour to grab at you sexually, and it is assault, sexual assault.
Sadly, many men are still deluding themselves that being a 'partner' (in name only) entitles them to ownership of your body. It doesnt.
Call him what he is, and keep on doing it.
Tell him its repulsive. Dont tiptoe around like some suggest, having a quiet word etc, its so gross and the ultimate disrespect of your worth.
You do not 'belong' to him. You are not his property to play with, his sex toy.
I would suspect a lot more about him, so watch his reaction, and be guided by it to make sure this doesnt escalate into anything else.