Glad that you have been seen promptly and have got the necessary medication for either potential issue! The sooner you can step in, the better any recovery chances. It is one of these issues, like eye injuries, where time really matters.
I hope that Gus is going to pick up again in the coming days as the medication is kicking in fully and is building up.
The weight is still a very good weight so I would not worry too much. The daily weigh-in at the same time in the feeding cycle is there to tell you whether your boy is eating enough hay or whether you need to step in with top up feeding. Keep in mind that hay is making around 80% and that you cannot control it by eye.
Since he hasn't lost his appetite completely, all you may have to do is to offer some recovery formula, mushed pellets or any mix thereof that goes down best; as much has wants to eat in one go if he is losing over 50g in a day. Unlike the poo output, the scales are not lagging behind and give you the status quo when supporting an ill piggy.
It is more the hot weather that is of concern for a poorly piggy with impacted appetite. Having had a very nasty middle ear infection myself in the wake of a really bad flu, I know how much chewing hurts or completely is impossible when the jaw joint is affected by the pain at first.
But Gus is having all the support he needs and will get through it!
All the best!