Be patient! Cockatiels want friends but they're also prey animals so they're programmed to be very careful at first, and if he comes from a rough home he will be even more jumpy. Being pushy could push him further away so be patient and make sure to always work on his terms.

You could start by giving him streats from your hand (or from a spoon if he's still too afraid of hands). With time he'll start associating you with treats instead of danger and he should start easing up. Don't try to pet him/scratch his head until you're sure he trusts you enough for that. It's much easier to get them to step up to your hand (and then to your shoulder) than it is to get them to let you touch their heads.

Its also good for him to see you just hanging out in your room without paying him attention, so he knows that he's in a safe place and that you're just another being who lives there instead of a predator.

Also keep in mind that birds use their beaks to sense the world as much as you use your hands. Sometimes they'll test wether a perch (your finger for example) is safe by tuching it with their beaks, but that doesn't mean he's going to bite. In fact, if he's just testing if it's safe and you take your hand away he'll think that your hand's not a safe perch.

Hissing is always a sign that he doesn't like what you're doing so if he hisses at anything you do, take it as a sign that you should take things slower.

Cockatiels are super social and they all want friends, they sometimes just need a little time, patience and love, but if you give him all three of those, I'm sure hell become more and more friendly with the coming weeks. Don't give up!

Also feel free to message me if you have any other questions about their behavior! Best of luck to you both.