Hannah, I'm sorry that your question wasn't answered in a timely manner. How old and which breed is Harry? Is he intact or neutered? His change in behavior over the last 2 weeks is worrisome but isn't pathognomonic (specifically indicative) of any one particular disorder. Rather than sending him home with a few pain pills I would have performed diagnostics in the form of blood and urine tests in addition to performing a physical examination. If such testing revealed nothing untoward, I would proceed to X-ray his chest and abdomen.
Is he still eating? Is he drinking any more or less than usual? Are there any other symptoms such as sneezing, coughing, nasal discharge, a change in his eyes, increased respiratory rate, regurgitation/vomiting, weight loss, swollen abdomen, or lameness? Are the mucous membranes of his tongue and gums still nicely pink or are they whitish (anemia) or bluish/greyish (cyanosis/hypoxia/lack of oxygen to his tissues)?