When Dad's not around

[sulphur_cam]

hey my family owns a sulphur crested cockatoo he is around 20years old i am 16 I have just recently started to be able to pat him while my dad is there tho while he isnt he always trys to bite my feet or shoes is this a tust issue if so any ideas?? I will soo be the one looking after him completley so i need to be able to get into his cage to feed him and change the water without being bitten thank you.