Do not modify the diet, just follow it precisely how it's to be given. Examine his fruit and veggie in the morning, you may find that he's eaten more than you think. Gliders will leave behind the parts (fiber) that they do not digest well. Look to see if some of the leftover food looks like a "shell", where you can tell he's eaten part of it.
Also, give him some time to adjust to this diet, if it is new, he may take a while longer before eating everything if he's not been fed well.