Multiple conditions - one PIP form?

Hi, apologies if this has been asked previously, I'm new and trying to work my way round the site and understand how to complete the PIP form......returning it by 4th Jan (received is yesterday!)

My adult son has returned home following a relationship breakup, he is ex RAF and was medically discharged, I think from memory this was due to his knee and PTSD, although they don't call it PTSD, think depression, something along those lines. He has also been diagnosed with Chronic fatigue syndrome, he takes tramadol daily for the pain in his knee, he is due to have an operation to remove some of the fat pad, that may or may not relieve some of the pain. He still suffers with depression and anxiety, some of that is forces related (Afghanistan) and some because of the CFS and constant pain.
I think my question is, do I list all of these conditions on the one PIP form? If yes, when answering the questions am I then relating the answers to specific conditions or combined as each condition aggravates the other. An example would be, if he didn't have CFS he would exercise more as that helps his PTSD, lack of exercise makes that condition worse, but exercise and CFS don't go hand in hand.