My cat has been eating about half of what he should

If your cat is not eating, like he was eating before, it's may be stress or anxeity.

Please try changing his food. Sometimes, all cats want is a change in taste. They get bored with same food everyday.

But make sure to consult your vet before making any change in your cat's diet.

I've also highlighted this issue in one of my blog post in details... Please check

Cat Not Eating Much But Acting Normal? (5 Surprising Reasons)

I see you have consulted a vet about this. If the cat hasn't had a full check-up, it's important to do that now, especially given his age.

If there are no health problems preventing him from eating enough, I do have one possible suggestion. Perhaps your cat feels torn between eating food and spending time with the people he loves. Being close to you may be more important than ever now that his brother is gone. So you might try putting his food dish near where you hang out in the evening. That way he can eat and be with you at the same time.

This may explain why he eats more when you're away and the cat sitter feeds him - he doesn't have to make a choice between being with you and eating.

I would suggest you to take your cat to a vet and do full his health scanning. Losing weight can be because of a lot of diseases: liver, kidneys, parasites, heart, etc. No one can make a diagnosis through the computer monitor.

If a cat is noticeably losing weight and you don't already know why: Vet. Now.