My dad lost 15 of his teeth. he's 65, so it could be due to old age or poor oral hygiene. should he wait for all them to fall to get dentures? should he get a teeth transplant (how much would that be)? or are there any better options?

He should be x-rayed and examined now in order to avoid additional problems. After a diagnosis and consult than a professional decision will be made as how to proceed. A oral hygiene program and healthy diet should be implemented. Implants are expensive as well as dentures but if he doesn't accept a healthy program than all measures will fail. What ever decision is made will be the best treatment.

Teeth are lost through tooth decay and gum disease, They are preventable. So it is a shame that Dad is losing his teeth. Waiting for proper treatment is a mistake because it ususally resluts in further bone loss. Well crafted dentistry returns the "biting chance at life". To give that to your Dad is going to cost some money whether for dentures or implant supported bridges. Go to a good dentist.