If you don't like him, then I highly recommend that you tell him you are not interested as soon as possible. The only thing worse than saying no to a date is saying yes knowing that you know you'll never be able to reciprocate his feelings for you. The mature thing to do would have been to turn down the offer from the beginning if you were unsure. But it's too late for that, so the next best thing would just be to let him know that you're not feeling up to it anymore as soon as possible before his feelings get hurt.