What should an elderly man do, who has had a stroke, cannot go to the bathroom, and urinates into a bag, regarding his ablution (wudu') and prayer (salat), especially if he goes to pray in the Grand Mosque (Masjid al-Haram)?

The Sharia has made capacity the basis of religious obligation, and among its rules is that hardship brings ease. If your father is able to perform ablution (wudu) and prayer (salat), even with assistance, then he is obligated to do so. Otherwise, he moves to tayammum (dry ablution). He is obligated to remove impurities unless it is difficult for him, and he prays in the condition that he is able to. He prays in the mosque unless his presence leads to its defilement or harm to the worshippers, in which case he prays at home.