I would think to start the pup on dry kibble as soon as possible, would be my guess. I know nothing about raising a three week old boxer so dont take my advice. How did you end up with just one? If you dont mind me asking.
I am glad that you are asking questions, but i will admit that i am a bit worried. You state that you have 5 boxers...two are puppies...and these two puppies are from seperate mothers. I know you are trying to learn, but i do have to ask whether or not you did any form of research prior to breeding your dogs. Did either have any form of health testing, did the stud? Is the fifth dog you have the father to both puppies. We are def here to answer questions and be as helpful as possible, but the breeding of our favorite breed is a very touchy subject on this forum as i am sure it is on many others. we dont support any form of unethical breeding, backyard breeding or hobby breeding. I am not trying to bash you, but trying to gain a bit of understanding as well as a bit of education. Whats done is done, but i do hope you dont plan on continuing to breed these dogs.
Normally you would feed dry kibble that had been moistened with a form or replacement milk or possibly water to make it a little mush and easy for them to eat. Once they are used to the mushy food you add less and less water to where they eventually are eating hard dry kibble.
I answered this question in your original post, but here it is again. Please note: When you make the puppy mush, do not use whole milk (cow) or goat's milk for the puppy milk replacer. Esbilac and Puppylac are well known puppy milk replacers.
At about 3 weeks of age, puppies will begin to imitate the dam eating and drinking. A secure shallow water dish should now be available at least part of the day. At 3
tootsie answered this quite well, which is why I didn't reply in the other thread, but you mentioned Weetabix here. I know lots of people do give Weetabix to puppies (and even to adult dogs), but being 95% wheat it's really just a filler; dogs can't process grains, so there's really no nutritional value in it for dogs. (For those who don't know the product, Weetabix is similar to Shredded Wheat, but made of flakes - similar to instant oatmeal - in a bar/biscuit form rather than shreds.)