Effects of chemotherapy

My husband had surgery for a Grade 2 Glioma brain tumour followed by radiotherapy and then chemo. He had 4 out of 6 sessions of PCV which he stopped as it was making him feel very ill, weak and he was losing mobility. It is now 10 weeks since he finished taking the pills and he still feels totally exhausted all the time, struggles to walk, not eating much and often feels as if he has a bad hangover. I know it can take a while to get over but we just need some light at the end of the tunnel. He is waiting for an MRI so we don’t know how the treatment has worked yet. Some days are better than others and I can take him for a ride out which he enjoys but why does it feel like sometimes he is going backwards and not a gradual improvement?