Hi! I am so sorry to hear about your pup.
I have lost a lot of my pups lately due to illness and their mothers dying.It
is a horrible feeling when you think you are going to lose them.I have lost 2
in the last day.
I had to handrear mine with KMR as their mother died when they were 10 days
old, even though your pup appears to be nursing it may not be getting enough
(pushed away by the larger ones) so you may want to supplement the feeding as
well.
I have another litter who were 14 days old when their mother died and I
placed oatmeal made with KMR in a shallow dish and the pups took to it
brilliantly.You could also try that, you want him as plump and healthy as you
can.
I am not sure about the bruise type markings on your pup but when my Mother
gerbil was dying she mutilated a couple of her pups.Maybe it is some type of
injury from the mother?
I wish you luck.
Sarah