Hello this is Dr Adil.
Thanks for asking question.
It seems that your Dad testicles get
injury during that event of falling.
Testicular trauma may cause
referred pain that feels in the abdominal area.
So some testicular pathology that has been resulted from the trauma event, is there.
Now all the tests came negative, it means there's nothing serious or something that may need emergency evaluation.
Now in this situation, i would suggest, pain killers,
cold packs and testicular support, as he is already doing.
You should seek another specialist's advice on this matter also, if problem worsens.
You may ask more question or give more information on this subject so that we have a better idea of the condition of your father.
Kind Regards.