Did the antibiotics make any difference to him? Sometimes vets give high dose for a few days, sometimes lower dose for longer, even for a few weeks... can you remember the name of the antibiotic and how long it was given for? If it made a difference at first but then he got worse again when the course was finished perhaps this is an infection after all. Sometimes a different antibiotic is needed.
In this country a vet will prescribe a painkiller called metacam to a guinea-pig that might be in pain, but I don't know if you have that option. I think it is only licensed for dogs and cats but its use for piggies is becoming more common here. If they feel a bit better this can sometimes improve the appetite. If his hay intake is down is he still able to eat any bell pepper or other vegetable to get his vitamin C?
I had a pig with neurological issues which caused loss of balance but he was much older (4 or 5) and it didn't stop him eating... in fact he was very cheerful and ate like a champion until the point where he started to have fits and sadly that was his end. It sounds like your poor little fellow is feeling unwell if he is not eating.
Have his castration wounds healed properly? Sometimes an abscess can form here causing swelling which might not be noticed. If you check him here does he look like it is causing him pain? (I mean, he's not going to love it even if he's feeling well, but you can probably tell if he flinches or squeaks). In your opinion as an owner, did he recover from his castration and then get unwell or has he never been right since that op. I'm assuming he had antibiotics after the castration.
If they are not eating hay properly for a few weeks their teeth can start to overgrow. This can be a secondary problem to whatever is causing his balance to go which seems to be the primary problem but it doesn't help things. (By the way, if he did not wobble around at the vet it might be an idea to try and video him to show how bad he is). Is he able to eat hay on his own? When you give him the Emeraid can you hear him chewing? And how much will he take over 24 hours?
I am not health expert - just a forum member - but I will tag
@Wiebke @Siikibam @Piggies&buns who might be able to say more. I don't know when people are online but I hope your little one can make it through. It's obvious how much you love him and are doing your best for him (ooh I see
@Siikibam is here - you are in good hands now)