Hi Ginger
Your son can mention it at the time of the assessment, and it will be up to the assessor whether they take it into account. The issue is, is not the atrial fibrillation (or any other diagnosis) it is how his medical condition affects the PIP criteria. You say it wasn't mentioned before - but did you not say that he had a heart condition or difficulty in functioning? Conditions have to be in place for 3 months (before a claim) and be an issue for the following 9 months for the DWP to take notice of them. He has nothing to lose by saying it.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems