Your father is lucky to be AB+ because that means he can receive transfusions of red blood from people with any blood type (A+, A-, B+, B-, AB+, AB-, O+, or O-).
For platelet transfusions, having a matched blood type is less important than for red blood cells, but in any case you can donate platelets for him.
How often you can donate is based on how long it takes your body to restore its platelet supply. This happens pretty fast so you can probably donate twice a month, maybe more often. The exact rules vary from country to country. You might check with the treatment center, or with a group like
Rotary Blood Bank, to ask what the platelet donation rules are.