Welcome! How long have you had your bird? It's true, cockatiels do tend to be very skittish. If you just got him, then he is still settling in and needs to learn in time that you are not going to eat him and there's nothing to be afraid of. It sounds as though he is not tame, so you need to keep in mind that you look like a predator to him. A giant monster. Take a deep breathe, a step back and remember: PATIENCE. Don't ask him to step up on your hand at this point, he is nowhere near ready for that. When you need to reach into the cage to change the food and water, do so gently and slowly, and talk reassuringly while you do it. Eventually he should stop panicking when you do this. As for giving him treats, he's still too scared to take them from your hand, so slowly drop one into his food dish from outside when you walk by his cage. Sit and talk to him, read aloud, or just let him see you going about your daily activities (on the computer, etc). Don't stare at him too much (it'll make him more nervous). Once he starts to relax,
then you can start trying to hand-feed him the treats, and just place your hand in or on the cage (not too close to him) for a few minutes at a time to show him that it won't hurt him. I would push the cage up to the back of a couch, or a table, and leave it open while you're around. Once he's started to feel secure in his cage, he'll probably want to come out and explore.
Again, I would wait to do any sort of step-up training until you see him starting to trust you more and not act so afraid. Hands tend to be the scariest part of us to a little prey animal, and even a bird who wants to be friends can sometimes have a very hard time accepting hands. I have a couple of tiels who will step onto a hand-held perch for millet, but they're not too sure about my hands yet (in fact, one is still terrified of them and I've had her four months).
The most important thing is to go at the bird's pace, even if it seems agonizingly slow to us humans. A week is like a nanosecond to them. Pushing them out of their comfort zone will get you nowhere. Back off as soon as you notice any insecurity or fear. Respect the bird's wishes and respect his space. You will be rewarded for it in the end.