My dad is diabetic he got big boils on all his toe fingers am worried what could be that? Is it very serious?

YES. Is serious. Probably his blood sugar level is too high. He can have the boils infected very easily and the problem gets worse. He should immediately be put under antibiotic therapy and have blood test. All under medical supervision.
Diabetic with boils. Because of the diabetes, it is advisable to take him to his doctor or a foot specialists to be evaluated and treat as this can become a serious problem.