If you know that it is ick, then you have 2 options, conservative and medication. Conservative is safer and not as harsh on the betta. Medications are not necessarily more effective but they can be easier depending on your circumstances.
Conservative: Increase temp to 84*F, add 1tsp/gal Aquarium Salt, 3 times, 12 hours apart so you end up with 3tsp/gal. If your fish is in something less than 2.5 gal then perform daily 100% water changes to remove fallen parasites before they reproduce and replace the salt. When treatment is finished, throw away the tank and get him something 2.5gal or more (2.5 gal is the minimum tank size for a betta). If he is in something larger than 2.5, then do daily 50% water changes. Continue the treatment for 14 days. If you cannot raise the temp then this method will not work, however, bettas are tropical fish so if you do not have a heater, purchasing one would help you treat him and increase his quality of life.
Medication: I highly recommend that you perform conservative treatment, but if you cannot, API Ick Cure or Kordon's Rid Ick Plus are acceptable medications. Do not use salt in addition to medications. Medications do not always work and you may have to repeat treatment. Increase the temp if possible. Perform daily water changes to remove fallen parasites and replace medication. Do not overdose.
Fish usually become susceptible to ick due to stress. In order to prevent ick in the future make sure you are performing proper water changes and keeping his tank temperature constant.
Under 1.5 gal: 100% daily water change
1.5-2.5gal: one 50% and one 100% per week
2.5-5gal: one 50% per week and one 100% per month
5-10gal: one 50% per week
10+: 30-50% per week
Bettas require constant temps between 78*F-82*F (water is always 3-4*F lower than room temp so unheated tanks are below room temp). Bettas are not cold water fish as petstores will lead you to believe.
Good luck and keep us posted!