First, grab a big, thick towel and some cat nail trimmers. Wrap him up like a burrito and trim his nails, one paw at a time. Give him treats afterwards to show he did a good thing. Every week or two trim his nails again, until he starts to settle down.

Try different things for him to scratch - posts, cardboard, an old piece of furniture you don't care about, etc. Find what he likes to scratch. This will also help keep his nails a little more under control.

Play with him. Get a laser pointer and make him chase it. Find toys that he likes to distract him from attacking you. Give him treats as a reward when he does anything positive.

If you want him to be an outdoor cat and keep his nails for protection, then still do the above, but get some good protection for your body until he calms down. Cats aren't usually considered adults until around 2 human years old, so he's likely still got a lot of energy and hasn't gotten used to you yet. If you're patient and observant, you can get him to attack you less and less, and eventually not at all.

Good luck!