Sorry to hear Jazz has been unwell. Am I right in thinking he is about 12 or 13? I seem to remember when Ben was around that age he had a fairly bland diet as he could no longer tolerate all the foods he previously had before. He had chicken and rice most days, a big spoonful of goats yoghurt or even live natural yoghurt (I know some dogs don't tolerate this but he always did), he would eat shredded wheat moistened with a little water, scrambled eggs, sometimes Weetabix but it does have added salt so we didn't give him that too often but he did like it!!
To be honest when Ben reached 13 I stopped worrying to much about ensuring he was getting all the nutrients he had previously and he had what he seemed to enjoy eating, he liked softer textures as he had some teeth removed and he continued to eat really well and enjoy his food. I used to worry he was missing out on things he had previously enjoyed, but if I did give him foods that were richer he just didn't tolerate them and either had an upset stomach or was sick so I kept it plain. I don't know how you feed Jazz, but Ben and Harry have always been on 3 smaller meals a day, and as Ben got older that seemed to work really well as he got a lot of pleasure from his meals and it was easy to give him any meds he needed with food as there always seemed to be a meal on the horizon!!