My grandad can't wear dentures because he has flat gums. What are his options?

Implants. Your grandad should get an evaluation for dental implants. He will need a minimum of 2-4 implants to retain n stabilize over dentures. His quality of life will improve exponentially. If more implants are possible, your grandad can have fixed teeth. This is the best option.
Dental implants. When there is no more bone to hold the dentures in place, you need an artificial mean to secure the dentures in place, dental implants are the best way to achieve it.
Surgery. Sounds like your grandfather needs to see a dentist who deals with implants and works with an oral surgeon for placement of those implants. Other options are more limited but possible but do involve surgery to deepen the floor of the mouth and the area between the lips and cheeks and gum ridge. Another option may be to seek the services of a prosthodontist.
Dental Implants . Hi, flat gums are a real challenge when it comes to dentures wear. The only available option for him are dental implants. He may might need to have bone augmentation procedures to be able to get them. Zygomatic implants can be an option for the upper arch. For the lower, if the anterior (front) part of the jaw has some bone, either the novum or the all in four systems can help.
Denture Adhesive. If he is physically, emotionally and economically able to have dental implants i agree they are his best option. Apart from that he can use denture adhesives to a limited degree. Soft definitive liners can also be fitted . These will need to be changed every 1-2 years but they can give him comfort. Remember he needs to take his dentures out at least 8 out of every 24 hours.