Don't jab down at him. This shortens your arms, gets him closer to your body, as well as allowing him opportunities to counter you 'over the top' with his right hand. Bend your knees a bit, widen your stance (front to back not side to side) and jab his chest. Hard to slip punches with your chest. Don't start your jab from too high- shoulder level, not from in front of your eyes. If you can find some tape of Bob Foster, watch it. He was very, very good at maintaining the distance he wanted to fight at.
Good luck.