Losing teeth. Depends on why he lost his teeth. Was it disease or was it from an accident or what? If from disease then treat the illness. Can he have dentures or implants? Meanwhile a soft diet with food mechanically softened so he doesn't have to chew the food is recommended.
Soft stuff to start. With tooth loss the best diet is soft foods to start. Get replacements of some sort soon to improve quality of life. I will tell you when i was a dental student i had a denture patient who got them only because his wife insisted he have them for going out. Without teeth he could eat anything. He could eat apples, steak etc. The human body is very adaptable.