I would suggest combining stroking him off with the blow job. Find that most sensitive spot (under-side, just below the head of the cock) and work that with your tongue...really pump your mouth up and down over his shaft, and alternate that with stroking by hand. Maybe finger yourself while you do it too and make the sounds that you only make when you are aroused - try anything to get him turned on visually, physically and with sounds...... Get him to lie back and close his eyes and just fuck the bejesus out of your mouth. Eventually you'll get the cum explosion you've been waiting for!