Sounds like you got a good bird... We have a 10 month old sun conure and he's done the same thing where he won't step up when he's perched somewhere he likes to be. Basically, the bird wants to stay on that perch or shoulder and he bites because he don't want to be moved.
If he's not biting super hard you can try ignoring the bite and convincing him to step up where he should submit and step up after realizing that biting won't keep you from stepping him up. If he's biting down hard tho, try offering him a treat he really likes, but make him step up in order to get it.
Sun conures can be very sweet birds, but they also go thru a stage where they get really beaky, it does pass ( kind of like a rebelious teenager trying to see what they can get away with) but it also needs to be adressed when he does this unacceptable behavior. Work with him using positive reenforcement.Show him the treat then ask him to step up. If he tries to bite, tell him "no no" or whatever word you use to tell him he did something wrong and turn your back to him for a few seconds... ours can't stand it when I turn my back to him. Then resume the taining by asking him again to step up for the reward. This works best when the whole family works these drills, especially when trying to get him to step off someone's shoulder, but you can also work him from his favorite perch and you can work with him alone.
The other thing that could be happening is that he might be developing a fear of hands, this can happen if he was ever grabbed by someone or chased around in his cage, or held him down to trim his nails etc... Basically just about any bad experience he may have had that involved someone's hand... this also takes a lot of work with positive reenforcement to convince him that hands are a good thing and only for petting, giving treats, etc... Getting a bird to overcome a fear of hands can be difficult, but not impossible, and it definitely requires lots of positive reenforcement.
One thing I teach all of our birds when we get a new bird is "gentle beak" training and you can teach this to a 4 month old bird as easily as you can teach a baby you just brought home. See my replies in this post for how to teach this to your bird, it works really well and can stop a bite on command...
viewtopic.php?f=11&t=11062 ,it doesn't work overnight and some birds take longer than other birds to learn it, but in most cases the bird can learn it in a week or less.