I've just stop breast feeding my son because he turn 1 two days ago how long does it take for the pain to go away? What can I use for the pain?

It will get better. It takes about two weeks. Try warm shower and take acetominophen (tylenol) or and anti-inflammatory such as Ibuprofen (motrin, advil) or naproxen, (alleve) this will help with the pain.
2-3 week of patience. Be patient! once the stimulation to the breast is gone, your brain is going to pick up on it, produce less hormones, and your milk will slowly go away. May take 2-3 weeks. In the meantime, if you are hurting from engorgement, you can manually, in very small amount, express your breast just enough to get some relief! be careful not to over do it as the torture my get prolonged. Use some advil (ibuprofen) too!
Cold turkey / gradul. There is no edict from on high that says kids have to be off breast @ 1yr. If your choice & kid willingly accepts chg then fine. Some kids/moms can cold turkey with help of binding/pain control over a wk or so. Some drop/substitute a feed every day or two till their engorgement deminishes then they have less intense pain & spread it over the 2 wks it takes to come off. Good luck.
Painful breastmilk. It should take a few days for your milk supply to diminish. Using an antiinflammatory like Ibuprofen or Naproxen sodium may help. Old fashioned remedies include cabbage leaves as compresses and/or ice packs. You could pump or express some volume to relieve some of the pressure. If your breasts are red and swollen see your doctor to check you for mastitis from clogged ducts.
Warmth; no coldturky. Warm soaks (?Tea bag warmed)on your breast, and pumping to relieve pressure (but not empty). Wean off more slowly.