I cant say for certain but my guess would be from eating some starchy foods or foods that contain polysacchardies (complex sugars). Some foods that have simple sugars like sucrose or fructose cause blood sugar (glucose) to rise very quickly as they are very quicly converted to glucose. Other foods like breads, potatoes, pasta, and other starchy foods take time to break down and get converted into glucose. Hence you may have eaten the meal awhile ago and your blood sugar may continue to rise for sometime afterward. The best thing is never eat a lot of those foods, and take smaller portions, and test yourself often. Drinking water helps too as it helps flush the sugar out of your system. However ultimately he has to find a way to control his diet so his blood sugar stays as level as possible and also consult with your physician. Hope this helps.