Hi Rocky! My advice to you would be to have him seen by his regular vet or take him to an Emergency hospital, whichever might come first. Usually the rule of thumb is that if your pet is lethargic, he/she should be seen by a veterinarian. My concern when I read this question is the lethargy, vomiting and diarrhea because to me that screams dehydration. So if nothing else, he definitly needs fluids. If he has a habit of getting into things he shouldn't then the doctor may also recommend an abdominal xray to see if he may have a foreign body. If his stomach seems bloated at all, which you haven't mentioned, then he needs to be seen on emergency because he could be bloated, which is life threatening. All signs point to him needing a veterinarian right now, there isn't anything you can do at home for him. If you'd like to take his temperature, the normal range is between 100 and 102.9.