Guinea pig constipation?

One of my guinea pig's with severe mange mites (being treated) has started to have constipation. I know this because I noticed big clumps of poo around his temporary cage. I read online that giving them apple peels and lettuce will help, but I'm not sure what to do? I can't pick him up because it only irritates his back causing him to go into a seizure. I know this may sound cruel.. but they aren't exactly my guinea pigs. I look after them for my older sister (25 years old, living with my family) who works long hours. I've fed him some lettuce and grass every day, but still no improvement. Please help :(