He looks very skinny, with sunken belly.
Anti-parasitic meds would be in order right away. Expel P (Levamisole) and General Cure/Paracleanse (Praziquantel and Metronidazole) would be a good first step.
Also, getting him to eat by soaking some food in Garlic Guard should be a priority. I generally like bloodworms or krill in this scenario.