jelly is a good one. get one of those 'add you own flavour' packets from the supermarket, instead of adding water or juice to the crystals make it up with hydrolyte and a tiny bit of orange or pineapple juice.
other then that i cant think of anythink beyond what your doing. but id maybe suggest trying to get a little in often, so offer a tiny bit of fluid in various forms once or twice an hour. maybe 50ml max at a time. he might be more inclined ti sip a bit here and there then be expected to down a heap at once? well thats what my kids are like when theyre sick anyhow