Cancer can be draining emotionally for everyone so you might consider this is not just physical. Make sure you take some time for yourself. This is not being selfish, its being a good caregiver because to be the best for him, you need to be the best for yourself. Take at least an hour a day to do something for yourself and at least once a week, several hours (like a day at the spa, or shopping or a movie). If he needs someone with him constantly, get a relative or friend to spell you or even hire someone if you can. You will be surprised at how much better you will feel, how much easier your tasks will be and how much clearer your thinking.
There are a lot of things you can do that don't require a great deal of effort too. Reading a book for instance. Something that allows you to mentally be away from the cancer for a period of time.