gregoryelliotj,
hungerstrike is often a result of stomach problems/parasites and can kill plecos.
I would suggest the following:
Keep the pleco in a quarantine tank that is fully cycled - having it in a fully cycled tank is critical as ammonia and nitrite buildup happens fast and has very deterimental consequenses.
Next do 20-30% waterchanges daily for about a week or until he gets better.
Raise the temperature slowly to about 84.
Don't feed or do so very sparingly making sure to siphon out uneaten food after about an hour.
When you do feed or he starts to eat , soak the food in some metronidazole - it's a safe medication against internal parasites and has no side effects. Seachems makes it and it's very handy to have with plecos.
As mentioned before, plecos need and love to chew on wood. If you can get a piece out of another tank that has some algae growth on it it would be great.
Do not feed beefheart, meaty foods or high protein!
Royal plecos are panaques and predominantly vegetarian, they actually eat wood too. Panaques have long intestins to extract nutrients out of vegetarian diet , by feeding them a high protein diet they often develop stomach problems and die.
Eventually when the pleco starts to eat and gets biggen you can add a little protein to the diet, especially when conditioning them to breed but you must make sure they have wood to chew on when doing so.
Plecos often get killed in shipping due to ammonia burn and oxygen deprivation. When collectors/exporters try to ship as many fish as they can in as little room (as shipping water costs money). The water gets contaminated with wastes, the oxygen gets used up , some fish ultimatly die in transport and the rest are forced to live in the same water as the decaying fish, this is where disease and parasites attach the weakened fish. Often the fish are dying when they hit the store and are sold.
I have had much more success acclimating plecos when using large 55-125gal tanks that are fully cycled. Don't feed the fish for the first 2-3 days as they are getting used to the new waterchemistry and often won't eat - the food just pollutes the water.
Good luck