All pups are different but at that age willow used to poop at least 6 times a day although she wasnt weaned on a very good kibble!!
I used to take her either out for a short walk or for a play in the garden before each meal that involved running about and this tended to get her bowels moving

You could introduce some raw veg to his diet this might help to loosen him and fill him up a bit at the same time. Does he drink much? Sometimes if they are struggling to go it can mean they need more fluid.