If I followed you correctly, you father's Given name in passport is Ram and Last name is Dhillon in passport.
If yes, fill all immigration forms with the following:
If asked for Given name in a form, fill Ram
If asked for First Name in a form, fill Ram
If asked for Middle name as well in any form, fill Singh
If asked for Last Name in a form, fill Dhillon
If asked for Any other names used in a form, fill Ram Singh
Use the exact Given and Last names as mentioned in passport to fill all immigration forms. Your father will get immigration with the name of Ram Dhillon
and if your father chooses to apply for OCI (once he has got Canadian Citizenship in future), he will not have any difficulty in getting OCI.