My dog whimpers when I touch his left lower belly. Is he hurt?

Poor guy. The left lower side of the belly is where the bladder normally sits, but unsure exactly where you are touching on the body without seeing or feeling myself. The exact location is key because certain organs lie in certain positions. However with just this symptom of discomfort from touching I would have to assume that an internal organ (perhaps the bladder) is painful. He should however have other symptoms like excessive urination, excessive drinking or foul smelling urine to corroborate the pain elicited during palpating the belly. If this is the case then a urinalysis and exam would be the best course of action to evaluate the urine and treat any abnormalities seen with appropriate and selective medication.

If the bladder does not seem to be the organ in mind, then a further evaluation by a vet to feel the belly is recommended, especially if your pet has any other symptoms of illness. However keep in mind, that if you notice any swelling of the belly or dissension and your pet is acting completely normal, then this should still be seen because organomegaly (enlarged organs), fluid in the belly or a growth (mass) is highly suspected and its best to look into this with a professional exam.