Take it to the vet and get a full work up on it . . . blood work, physical, check the teeth, check the thyroid (blood work), check for lyme disease and other similar diseases, if the vet thinks its neccessary, x-ray hips, elbows, knees, back.
If his hips are aching that could be why he is getting easily aggitated. Putting him on a joint supplement and possibly a pain killer might help (again, consult a vet). Also, older dogs often have bad teeth that need to be pulled. Most dogs when they have bad teeth are VERY cranky, as are most people.