The next stage of jealousy is the stage at which you start feeling sick to the stomach, losing sleep and going through his pockets looking for proof he's playing away - whether you have a valid reason to worry or not. It starts to become an obsession.
You grill him about everything: (Where were you, at what time, who were you with, why?) and you refuse to believe a word he says.
This kind of jealousy makes both of you suffer: you tell him he has to prove he loves you, you start setting ultimatums, and it'll all end in tears.