It may be that he has an anal fissure, and his bottom is bleeding a bit from pushing so hard. Are you seeing the blood in the toilet, or on the toilet paper?
Blood from the stomach will look like coffee grounds and be blackish. Unless there's something going on in his intestines, check his bottom. My son has a little bleeding from time to time. We use Derma E ( a tea tree oil based cream with vitamin E) to keep the area clean and ensure against irritation.
You can also try changing his diet to encourage a softer stool. Think blueberries, apples, pears...anything with good fiber. If you decide to try dried fruits, be sure to stew them first, otherwise they'll just be absorbing more water out of the colon resulting in even harder stools. Steel-cut oats, beans, and other foods with roughage will help. Limiting processed carbs and replacing them with whole grains really helps, too. And cut out any caffiene, which is a diuretic. (This means cocoa/chocolate milk.) Limiting dairy will also help.