I am betting that it's a combination of these 3 things that make your dad more appealing to your bird than you:
A) More confident
B) Positively Reinforcing
C) History
I could be wrong about these because I have heard very little of the background information but that is what is sounds like to me. However, I suspect that the bird is gravitating toward your step dad because he's not afraid of the bird and handles himself confidently. You're asking us for help rather than just going for it, so that implies that you are more careful and concerned.
Next, I think something your step dad is doing is more positively reinforcing. Cockatoos are very social birds so it is possible that the social interaction your step dad provides is more rewarding. See what he does and try to do more of that. Just because you clean the cage and spend more time with the bird does not mean it will like you better. It has no sense of appreciation for those sorts of things but mostly just cares about what it can get from you at the moment.
Finally what I mean by history is it sounds like this started happening more recently because you've said you had the cockatoo for 3 years. It seems like the bird has gotten used to you and knows it can get away with it. It probably knows what to expect and knows you're not gonna do much for it so it doesn't try.
I just reread what you wrote and noticed: "i open his cage and let him play around the outside of it everyday and he climbs back in on his own."
Yup, this definitely implies a lack of confidence on your part and the lack of positive reinforcement. You need to toughen up and be more proactive in your interaction and also you need to create situations where you are in control simultaneously where the situation is rewarding to the cockatoo.
For starters, you should check out
my article about how to teach parrots to step up to come out of the cage. Secondly, you should observe your step dad and ask him to show you what he does with the bird. I bet you'll have a bit more success if you do what already works for your step dad.