Possibly the easiest thing to do is to just follow the jab back in with him, especially if he throws it lazily. Just watch out for him doubling up the jab, and always keep an eye out for that follow up cross. Also, depending on the type of boxer he is, see if he tries to block a feint or two, but I wouldnt rely on that too heavily (he should be pecking away from a distance with the jab, not covering up, but hey...its worth a try)