Just give him some time to settle in and he should color up. New fish may take up to a couple days or so before they feel comfortable enough to eat.
If he doesn't eat after that initial phase and he is getting skinnier, I would suspect internal parasites. Stress is still a possible reason, whether he is not comfortable in his tank for environmental reasons (hiding areas, water parameters, tankmates, etc) or he may not be feeling well from some other sickness (doubt, but possible).
You can treat parasites with Praziquantel (found in Hikari PraziPro) or Metronidazole (found in Seachem MetroPlex/Aquazole). Would probably be necessary to treat the water column rather than use medicated foods if the ram isn't eating. Safe enough meds to treat the whole tank.