Neutered male cat never wants to be indoors

Our one year old (neutered) male cat Zahrim is barely ever inside with us anymore and we not only miss him, but worry for his health. He has irritable bowels, needs a special diet and medicine once a week and we keep up with worm and flea treatments. He is underweight and getting him to be home to eat is a battle. We've tried a lot of different methods to entice him to eat, but he only takes a few bites and immediately wants to be outside to the point he is meowing and whining constantly at the door and scouting the house for any open windows. My partner and I have already spoken to the vet about his behavior but aren't getting any results. We never had this problem with him while he was growing up. He still loves us, is very affectionate and happy to have our attention and will even sleep with us on the rare occasion, but can't seem to stand being in the house. Sometimes he will sit outside and whine for us to come to him. I've noticed he hangs around the house and the neighbors house (which she doesn't mind at all)

He used to have a brother and they were very close, but at 9 months old we lost his brother in an accident. But Zah's absence from home began a month before his brother passed.

About a month and a half ago my Mother saved a kitten from drowning and put her in our care since she had too many animals and no room for a kitten. This kitten is roughly 10 weeks old and hyperactive. The two had a shaky introduction, but now Zah doesn't mind her much at all. Sometimes he'll be spooked if the kitten is galloping around the house at full speed, otherwise he's alright but again he's barely ever home. He'll make a pit stop at about 4am for food most days and be out again straight away.

We've tried locking him inside but we always relent after hours and hours of non stop cat-whining. I feel so guilty making him stay inside when it makes him so unhappy, but I have no idea what to do. I've googled and read forums, but I can only find explanations for the cats behavior and never any possible solutions to the problem. Any ideas? Thanks so much for taking the time to read.

Lots of catnip. I actually brought a Tomcat in after I noticed he was getting constantly beat up by a new Tom in town. He lets me pet him but never pick him up so he is still unnutered. At first yes he whined and whined and would bolt out the door but eventually it goes away over a time. Drowning in catnip helps. I made sure he had an alternative to going out by giving him a couple window perches and a stool to sit and watch outside in my screen room. Because mine was unnutered it took about 2 years but he is still with me now 5 years later and now doesn't try to bolt out the door or yelps to go outside. I have adopted a few I was able to neuter and it is much much easier. The age of the cat matters significantly as well. The older they are the more they hang around the house. An indoor life is a much safer one try to stick to it. Once he knows yelping won't get him an exit he will stop doing it. He has probably trained you to let him out when he starts doing it and know he knows that

Big thanks for the advice and wisdom! We've decided to keep him inside despite the meowing and I am on amazon getting the catnip as we speak
It's great having the forums here for support, I feel like I have more of a firm direction to go in with him now. Thank you Hexiesfriend.

No problem glad I could help.  In the end you know he is safe and sound at home.  This is my cat Tom, who has taught me a lot. Though he used to be rough and tumble, he has, as you can see,  gotten used to the finer things in life !!!!

Getting him a window perch or cat tree to watch the outside would be helpful too.  You could even set up a bird feeder near the perch or cat tree for him to be entertained.  I brought a feral/stray into my home in late May.  When first came inside he would throw himself at the window to try and get out.  It took him awhile to settle down.  Then when he was allowed downstairs near the doors, we had to be really careful.  I held firm and decided that he would never go outside again.  A few things that helped were feliway plug ins or feliway spray.  I also used feline Composure treats and/or Composure liquid max.  Both are calming agents.  The treats can be given 3 at a time.  They take up to an hour to work and last about 4-8 hours depending on the cat.  The composure liquid is put in their wet food and works in about 20 minutes.  It last about 8 hours.  The liquid is more expensive, but works better in my opinion.  It helps to calm the cat, not drug the cat. 

Good luck with your task.  I hope you are able to keep him inside.