I dont think that there is much that you can do, with him being so young. Just keep feeding and keeping him warm. Did he not start suckling from the mum? or are these all being hand reared.
He may still have fluid on his chest or an infection if he is not breathing well.
You could take him along to get checked over, but at this age it is difficult to prescribe anything.
I would prepare your self for the worst, but if you feel that he is not feedin adequately or is having problems breathing and is struggling generally i think you should consider whether it is fair to keep him going, and consider having him euthanased.
It is so distressing to see them in discomfort.
Selena