BJONES
There are a couple of aspects to this.
Yes you should explain assistance that he requires in order to complete the PIP activities.
He is not required to complete them pain free, but any pain that limits his ability to do so should be considered.
Have a look at the use of "reliably" and "on the majority of days" in the PIP Claim guide, as these may offer you other ways to explain his restrictions.
If you have further questions then please reply to this post and we will do our best to help.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems