I read your question and I understand your concern. Your father in law has two major risk factors for vascular disease like high blood pressure and diabetes. Also I am assuming he is of a certain age which is also a risk factor. The symptoms you are describing could be indicative on an occlusion of a vessel in his left limb. It usually has other symptoms like lack of pulse, cold skin if an artery occlusion or swelling if a vein. Another possibility, especially if you notice asymmetry in his face or involvement of his lower left limb (even if less than the upper one) is a cerebral involvement like stroke or with that progressive evolution a tumor as well. At times a heart attack could manifest with numbness and tingling of left arm, but usually other signs, more dramatic event, less likely. Another option, less threatening, is cervical disc disease or peripheral nerve compression. Anyway he requires a physical examination to guide towards the most probable option and following confirmatory tests, so since some of the possibilities I gave could be dangerous I recommend to see a physician sooner rather than later. I hope it s nothing serious and things work out for the best.