Oh my Goodness! If he isn't the cutest freaking puppy I have seen in a long time!!! I love him to pieces with his little shoe eating, and his belly showing floppyness!!
I wouldn't worry too much about his eating habits, if his energy is good, and like everyone else said, his poops and skin are healthy then he probably just doesn't
need to eat as much right now. When Naja was at that age she was a bit of a light eater, then 2 or 3 weeks later she was like the garborator from h*ll! lol. Just wait for a good growth spurt and see how much food your going to go through. His food intake will go up and down with his growth and energy levels. It also sounds to me that he might be waiting for your Jack Russel to let him know it's okay to eat. You said that he waits for her usually before he starts. He could be just following the pack order of who eats first as well.
BTW if he goes missing, don't worry, I have just kidnapped him.