Hi I'm Rosie one of the vets and I'd like to try and help you. That temperature is quite high for a cat - it should be about 101.2 in a healthy cat. When cats have a temperature like this they feel like we do when we have the flu - they just want to go to bed, curl up, and forget about the world. He will be feeling fairly rotten with this - hence his lack of appetite and lethargy.
The most common reason for a cat Milo's age to have a temperature is an infection, so the antibiotics should help. However, I am slightly concerned given what you have told me, and I think it would be a good idea to get Milo back to your vets. I am assuming that because your vet gave him fluids under the skin that Milo was dehydrated at his appointment, and if he isn't eating now this dehydration will only get worse. This will make him feel worse, and reduce the chances of him eating - it tends to cycle after a point. You can tell if he is dehydrated by feeling his gums - if your finger sticks to them, he is dehydrated. If he is dehydrated then he will need more fluids from your vet - another injection under the skin is a possibility, but ideally he would need to have some IV fluids (go on a drip) for at least 24hrs.
Cats also don't do well when they don't eat - they try and use their fat stores for energy (instead of the food
they haven't eaten, if that makes sense) - and this fat overwhelms the liver and causes problems. This is potentially a serious condition to develop. The best way to get him to start eating again is to take his temperature down. As you haven't mentioned it, I am assuming that your vet didn't give Milo an injection of anything to do this. Unfortunately, cats are really sensitive to human anti-inflammatories so you absolutely cannot give him anything at home.
Normally my treatment for a young cat would be an injection of rimadyl to take the temperature down, followed by a course of antibiotics. With a temperature like Milo's I would want to recheck them the following day to make sure the temperature is coming down. Like I said, if he is dehydrated then he will need more fluids. You cannot give rimadyl to a dehydrated cat, so he would need fluids before the injection. I would talk to your vet both about fluids and an anti-inflammatory to get him eating again.
Like I said earlier, given what you have told me I would get him back to your vet asap for the above treatment, plus whatever your vet feels is necessary. The good news is that the vast majority of cats with symptoms like Milo's do just fine with treatment.
I hope this helps - if you have any questions from this then hit 'reply' and I will get back to you. Otherwise, if you have found this useful, please hit 'accept'. Good luck, Rosie.