If it's been more than a day or so, you need to intervene more directly. Soak a tea towel or pillowslip in lukewarm water and wring it out, then wrap up your snake in this and hold him in it for a while (perhaps between commercial breaks on TV). Then let him wriggle out, hopefully removing the retained shed. Use the damp cloth to gently rub off whatever is still left on his body. Be sure to get every bit off - especially on the tip of the tail. Next time he's coming into shed, spray the enclosure with water daily from the time his eyes cloud up until he sheds.