You will need to keep the area around his butt very clean and trimmed. The poo is getting stuck and blocking the passage which can cause serious problems. Check him dailey and plan for daily cleanings
At 6 weeks old he should be getting kibble and even some canned food. I take kibble and grind it up in the blender and leave that for my pups along with plain kibble at well. You can also warm some goats milk mixed with some water into the ground kibble.
He definatley needs more then formula at this stage. If you have any questions let me know and I would be happy to help. I have 6 week old pups now and they are still nursing and eating what I just told you. At 6 wks they should still be with mommy.