Rule number one with keeping fish, if there is a problem change 50-75% of the water with clean, dechlorinated water! The solution to pollution is diltution. Although if the other fish are well it may not be related to water quality. Changing pH for gold fish is often unnecessary. He may have a parasite which at this point there is little you can do to help, if his abdomen is very bloated and scales are sticking out it could be dropsy (renal failure caused by bacterial infection) you could try adding a broadspectrum antibiotic to the tank although this oten involves adding medication and changing water over a number of days which is going to be difficult if no one is there to do it.
Have you added anything new to the tank? If so take it out in case it is contaminating the water.
Goldfish can live for over 30 years if cared for properly so its unlikely to be old age but if he had a really rough start to life and lived in poor conditions it may have reduced his life expectancy.
Unfortunately if he dies he is unlikely to traumatise the other fish, as horrible as it sounds fish will often eat other dead fish. If they dont his body will pollute the tank as it decomposes and potentially kill your other fish
If he is still looking poorly before you go you can euthanise fish etiher through your vet, using clove oil in water (ggogle for the exact ratio of water to oil) or by putting him in extremely cold water then freezing (apparently they metabolicaaly shut down and dont feel pain with these methods but who knows). lushing a dying fish down the loo can be very traumatic for the fish.
I hope your fish makes it, unfortunately when fish show signs of illness it is often too late to save them but changing water etc may prevent other fish in the tank getting ill too.
ETA if you are prepared to pay there are companies that will come in and look after fish while you are away


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