The growl is a good thing as it means your boy is at least vocal about his feelings (better that than subtle body language prior to biting

IMO). The question becomes how annoyed is he, my male Gizmo will growl at the slightest annoyance, so pushing the point generally wouldn't necessarily result in a bite, my female Pebbles rarely growls, and if she does, you are doing something that is really pissing her off, she has never bitten me, but that's probably because I have respected her when this has happened. So the most important thing is to get to know Sully, and his personality. As already suggested some target training in the cage could be a good thing. Although I agree my two don't get treats anymore for step up, I wouldn't be afraid in the beginning to treat away, as eventually he will step up because he knows that's time with you (which is the best treat), but the food treat is the substitute until he realises how fun it is to be out of the cage with you.
As for when and when not to push, that is a hard one to answer, as it really depends on a lot of factors, and this is why getting to know Sully and him to know you is the important step. Your right to have a healthy respect for his beak, but like with dogs, parrots will pick up on fear and hesitation, this is likely to cause mistrust, so make sure you work through that. The reality is you probably are going to get bitten at some point, that's just the way it is, especially if you are new to parrots. The key is not to let the fear cause the problem, for example, you don't want to be thinking is he going to bite me and flinching your hand when he is just trying to get a good grip with his beak to step up.
To explain what I mean by the importance of getting to know Sully, I have effectively three step up commands. There is "step up?" to which Gizmo and Pebbles generally say yes please, although some days Pebbles just wants a little time in her cage before comming out, and that's fine, I will give her some time and eventually she is at the front of the cage saying "hey let me out"

. Then there is two forced step ups (when something is wrong, or I have to go out urgently, etc.) With Gizmo it's about attitude, a slightly lower tone, still happy, but with purpose, and moving the hand towards him with confidence. With Pebbles, it's basically a case of a two handed step up, one hand on each side and I move them in towards her as I say step up (again in a slightly lower tone, with purpose). These are not interchangeable, using Gizmo's technique with Pebbles would just bring out her stubborn personality and she would either sit there holding on, or turn it into a game of catch me if you can. Using Pebbles technique on Gizmo would either see him fly off, or bite you. The reality is, if they wanted to, I would have no chance getting them back into the cage, short of having a net, especially Gizmo who is a very talented flyer. This is where the them knowing me part comes into play, they know that they will get to come out again, they know I generally do give them a choice, and they know with this tone I am not going to give up.
With all this said, don't stress too much about what's right or wrong, or getting him to come out of the cage, you will hopefully have a whole life together to figure each other out. I am certain before too long Sully will be dancing at the front of the cage first thing in the morning, waiting for you to bring him out