If you can catch him...fw dip him...then put him in a QT with anti-biotics to prevent/treat secondary infections/injuries caused by the ich...
When you fw dip him...ensure the water is dechlorinated and at the proper temp...keep a close eye on the fish, and do not exceed 3 minutes or so...
As mentioned..the fw dip is stressfull on the fish...the dip does a few things for the fish...it removes the cysts from the gills and internal mouth as well as exterior cysts...it is an immediate removal due to the cysts basically rupturing from the rapid absorbtion of the fresh water...osmotic pressure...
The fish should then be put into a tank with an anti-biotic...with new fish with ich...I myself use amoxicilian 500mg capsules, I have found this to work extremely well, for about 8-10 days.
Keep in mind, most users on this board advocate the hyposalinity stuff...I do not as it is time consuming and is basically accomplishing the same thing as a dip without the stress.
A good note though...I have never lost a fish to a dip yet...and saved a few with the combo fw dip/antibiotic regime, whom I doubt would have made it...