Bloat or an obstruction would be the main concern, and would be considered an emergency. It is possible that since he got into some old trash, he may have gotten into something that didn't agree to him, or potentially contracted a bacterial infection.
If it is possible, it's probably safest to get him into the emergency clinic tonight to have him evaluated and rule out the possible emergency situations such as bloat or an obstruction (like if he ingested something else from the old trash that isn't passing through). He is within the right time frame when we start to see signs of obstruction, and that can include slight bloating, lack of appetite, nausea and vomiting, restlessness and inability to lie down due to discomfort, straining to pass a stool, and lethargy/tiredness. The signs also seem similar to what he's describing know. An obstruction can be life-threatening, so the sooner he gets treatment, the better.
If you are absolutely unable to get him into the clinic tonight, pull away his food and water for the evening, and get him into the clinic first thing tomorrow. If he starts to vomit profusely, if his gums grow pale, or he grows unresponsive when you're trying to rouse him, those are signs that he's in more immediate danger.