With a sick pup I would hesitate to change his food or to try a lot of different things. One thing you want to do is to let his tummy rest. If he was my baby, I'd try to find one thing that he would eat and stick with very small portions fed frequently - every hour or two and encourage him to drink water until his stomach settles. Then I would start back with the kibble mixed with his "sick kid" food. It would help him over the hump by providing enough calories to prevent hypoglocemia plus ease him back into eating his kibble.
To help encourage him to drink - try a very small amount of white corn syrup (kayro) in his water to sweeten it a bit.
Also there is a fantastic product called DYNE - you can order it from Revival -
Dyne - Revival Animal Health. This is a high calorie supplement that will help boost his calorie level until he is back on track with his eating.
Sending healing prayers for Ritz and comforting prayers for you!