Greetings from Georgia,
I am a dogman from way back and have seen this situation a number of times. Seperating a dog from its mother early can be traumatic no doubt, BUT, it can also lend to some very deep bonding with the dog's "new pack". All dogs are pack animals in their genetic makeup, some more than others. A Beagle is known to be raised and hunted in a pack. JRTs are a bit more independent, but still prefer to be with their own "family". The most important advise I could give is to get him to your vet as soon as possible. Shots are needed and I am sure that your vet will be able to answer most any question you may have. One thing I have used regularly to help my dogs pick up weight is regular calf starter you can get at the local feed store. It has done well for me as well as other dogmen I know and I have never known of any negative effects.
Now, as far as the training goes, I posted here earlier about a sure fire way to train a dog. I still reccomend this, but with a young pup, it would be best to get all his shots completed first. This will also give the dog time to grow a bit and find his place in the family. You can begin a lot of training now. Basic obedience is PRICELESS !!!! This will make a dog a joy to live with and the lack thereof can drive you crazy. If you are willing to put forth the effort it will be repayed a thousandfold. I will share a few other proven tips that may work for you. Never allow a pup to do things you do not want the grown up dog to do. EVER !! Place one of your worn t-shirts in the dogs bed, and change it every few days. This will help to imprint your scent into the dogs very being and he will never forget you. Take the dog with you even if it is only a short trip. Hey, dogs like to go out to. Always be fair and consistant, as this will help prevent any resentment from the dog. I guess most of all, just love the dog, and remember, right now he is just a baby and does not know any better. It is up to the leaders of the pack to show the new member what is expected and accepted. Good luck with your knew pup, and keep us posted.
Dave