Update: the little one has passed.

I have a baby who has been with me for a week and is not thriving. He is kept warm, was given electrolytes initially and was eating ok for the first few days. I estimate him to be about 5 weeks and am feeding fox valley 32/40. He eats well his first few meals and by evening is always less interested in food and pretty lethargic. His cage mate is doing very well. This morning i found him with diahrrea and thrusting his head back, doing the open mouth fish blowing face. I put some apple juice on his tongue and gave him 1 ml sub q fluids of lactated ringers and kept him warm. What should my next step be?