I would combine advice given to you by others. "Henry, my schedule has changed. I can still take you to the grocery, but I can only help you for X amount of time. I know you enjoy these trips, so I need you to help me in order to help you. You'll need to write a list of things you need before I pick you up." If he balks, tell him point blank you will not shop with him without a list so it's easier on both of you. Then start making note of brands that he likes. If he expresses that he likes Dole canned pineapple, when you get to that aisle, say something like: "Hmm, you have pineapple on your list, we're in the canned fruit aisle. Here's the Dole pineapple, just like you had last time." If he insists you stop to read other labels, say "We don't have time for that, Henry. What's next? Cereal? All right, this way." When he tries to get you to read labels, refuse. "You've gotten Y brand of this before and you liked it. We'll get this, and if you really want to know how much sodium is in it, you'll need to ask someone else at home because we don't have time to read everything right here."
If he hasn't expressed a preference til now, oblige him with reading labels ONE TIME, and when he picks one that is acceptable - make note of brand somewhere, maybe on the shopping list itself. On the next trip, ask him if he liked the (whatever). Yes? That goes on his acceptable-brand list; if no, skip it when you get to that section. "Henry, you said you don't like this brand. This one is pretty close to (whatever criteria he has). Let's try this." This counts for items he gets only infrequently. Eventually you'll have a list of acceptable items, which both helps you pare down shopping time and label reading, but also 'standardizes' things for anyone ELSE who might have to shop for him. Snowstorm, you're stranded across town, neighbor offers to go pick up a few things - Henry can say, "I like Z brand bread, B canned mixed fruits, K pasta..." and know what to expect when those items arrive.