There are many things you could do here, feint the jab, double the jab, or as stated before, you can jab to the body, or feint to have him throw his jab, than slip under it and dig a straight to the body... Also, have you tried letting a quick 1-2 go? Make sure you arent overcommiting and telegraphing the punch as well. I definitely wouldnt abandon the jab though, you should keep it busy even if its not landing to set up other shots if nothing else.