no wonder he's so skinny & lethargic & I don't mean this instead of going to the vet. he really needs to go.
at 12 weeks he should be getting fed 3 times a day, but since he is in such bad shape I'd give him four. he is a baby & yes he needs to go out every hour or so at this age if you have any hope of getting him housebroken. you don't starve the dog to make it easier for you.
by only feeding him once a day you are risking his glucose levels dropping so low it can cause seizures & death
the reason he poops so much is because the both the foods you are feeding him are garbage & full of fillers which go straight though the dog to your yard. there is very little nutrition in either one.
have you been withholding water also? puppies dehydrate very quickly & can die from this as well.
as for food - nothing from the grocery store or WM. no corn, wheat, soy, by products, artificial dyes or preservatives & lots more. here is info for you to learn about food ingredients so you can make some better choices for this poor puppy.
Dog Feeding Info
do not give him large amounts at a time. your vet can tell you by his weight how much to feed him.
you can add some cooked rice, noodles, pasta, cottage cheese, boiled white or sweet potatos to his food each time - a tablespoon or so each time.