I wouldn't withhold food. You can add about a tbs of canned pumpkin to his food but if he's not better by tomorrow morning I would be calling the vet for an appointment. If you haven't changed anything in the past few day (you said 2 weeks ago with a food change) having a change in poop habits would be of concern.
If you are going to try adding a little pumpkin it needs to be Canned PACKED Pumpkin. Not the type that is for pie filling. It's pretty cheap around $2 for a can and you can always freeze any left overs for use later.
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