Being helpful when we can is always a good thing. Unfortunately, some people take advantage of those who have good hearts. You would not be wrong for refusing to run errands for this man. I'm assuming you are not being compensated in any way for fuel, time, and inconvenience. If this is the case, it is outrageous that you are even being asked to do this for him in the first place. It might be different if it were only one time, however, he seems to be depending on you regularly to take care of his business.
If you no longer wish to deliver his groceries and/or other goods, you have every right to stop. Don't let him or anyone else guilt you into thinking he is going to starve without you, because he has obviously gotten along just fine before you came along. Just tell him, "I'm sorry I can't be more helpful. Unfortunately, my schedule is just too sporadic and busy. Perhaps you should look into the delivery services and community resources available in your area so that you can get the help and assistance you need, as they can give probably give you much better service than what I am able to give you."
If you don't mind helping him once in a while, set some ground rules, and see to it he respects them. He doesn't get to negotiate. Your time, your money, your vehicle wear-and-tear, your inconvenience, YOUR TERMS, not his.