The usual escalation path is as follows - In the USA, at least. I am assuming you don't have a union. If you do... well... bug your union rep, they know your work better than the internet does.

1. Talk to the co-worker and ask them to stop. You only need to do this once.
2. Talk to your boss and give him an opportunity to end the problem. You don't need to gather more evidence or do his investigation for him. The fact that he's busy doesn't matter.
3. Talk to your work HR department. Likely they know their legal obligations to prevent workplace harassment and are going to be pissed at your boss for sitting on this for a year.
4. Talk to the EEOC directly. File a claim that you were sexually harassed and nothing was done and they will come knocking. You don't need a lawyer to file a complaint.

The usual escalation path is as follows - In the USA, at least. I am assuming you don't have a union. If you do... well... bug your union rep, they know your work better than the internet does.

1. Talk to the co-worker and ask them to stop. You only need to do this once.
2. Talk to your boss and give him an opportunity to end the problem. You don't need to gather more evidence or do his investigation for him. The fact that he's busy doesn't matter.
3. Talk to your work HR department. Likely they know their legal obligations to prevent workplace harassment and are going to be pissed at your boss for sitting on this for a year.
4. Talk to the EEOC directly. File a claim that you were sexually harassed and nothing was done and they will come knocking. You don't need a lawyer to file a complaint.

If you don't get to step 4, great! It's been resolved! If you do, then you have ample evidence that you've given your work opportunities to resolve this, and they'll be able to drop the hammer.