I would recommend the BRAT diet as someone else mentioned. Also, there is a powder product or chewable if he would chew it, called Babydopholis (sp?) which is acidopholis like you get in yogurt. It really helps a lot in getting their systems back on track. If you go with the powder, you can shake it up into a bottle of milk and he will never know w hat he is getting. Also, their is a bottled water on the market by Glaceau called Smart Water. It has electrolytes etc in it just like gatorade or pedialyte except with out the sugar. Perhaps you could make some jello water out of it to get him to drin, or if he is still on formula we used to make our twins formula with it when they were having issues like your's is.
Will he go for popsicles, otter pops, jello or even just making the jello and letting him drink it before it sets up (my mom is a nurse and they used to do that many years ago) Applesauce is usually easy to get in them, or toast with a little somethign on it to flavor it, or even cinnamon bread toasted. I would suggest to just find something he will go for and let him eat as much of it as he wants (along the BRAT diet) so if he wants only applesauce for a day or two, go with it :)
Good luck!