Issue down there...

So my boyfriend has an issue. Throughout the day his penis gets hard from my constant teasing, touching, and kissing him. The problem is when it comes down to us having sex his penis doesn't want to stay hard. No matter what I do, no matter how long I'm giving him a blow job, it doesn't get hard. It's frustrating to us both because he denies having any problem & I'm not be sexually satisfied. Does anyone know what might be possibly wrong?