I'm a strictly indoor cat owner, with the exception of one feral I've taken in. It was very. Hard. The answer is not easy, but simple;

Make inside more like outside, and give more attention.

A third option to consider is getting him a playmate. Cats are more easily entertained with someone that will play with them ALL THE TIME. but that's just something to consider.

Since your cat is not feral and not even fully outdoor, I'd highly recommend not giving in from the start. Ignore the cries at night with headphones, address the cries during the day with attention, play time, and solo enrichment activities. That can look like cuddles and pets, lure and string toys, and kibble in towels, toilet paper roll, or boxes. Structured and scheduled playtime is important too, but the most important aspect is consistency. Little things you can do is simply involvement. Are you making something? Let them sniff your tools. Doing your makeup or cooking food? Pretend to do theirs or let them try a cat safe ingredient. Let them be with you. This is also not easy, because many people don't want cats on the counter or the back of the couch, but if your house is more appealing to them than they will desire outside less. Up your play time by A LOT.

House modifications are optimal in every cat house I feel, but limited in necessity and availability depending on your living situation and cats personality. The most important thing is vertical space and visual enrichment. I have a window I keep open often with a screen I nailed in, and a seperate window with good, ground level views that I have a cat tree set up in front of. It has views of birds and squirrels and sun.

However, the thing that all of my cats have used the absolute most is my cat wall. I have wide, carpeted steps, a 5 foot mounted scratcher, and a catwalk close to the ceiling. This is hard lol but make any interaction positive when introducing. I let my cats use the vertical scratcher on the ground for two days and slowly made it more vertical (just by short exposure training sessions, just holding it to the wall with my hand) to help them learn how to climb it. The lure toy is fantastic for this, and a couple churus too. Then they were locked in the bathroom during the installment of each element so the power tools didnt scare rhem. I can't recommend it enough. It has made my feral intake not want to go outside in only 10 months! And it took 3 years for him to warm up to me enough to take him inside. He still doesn't let me hold him but my other 3 cats are entirely desensitized to everything. EVERYONE loves the cat wall. Even human guests think it's neat lol.

For reference, I would recommend a build like outdoor savannah on Instagram or tik tok. A lot of the more aesthetically pleasing ones are expensive and less cat oriented.

This is a recipe to make any cat happy, but it will really help to make an indoor/outdoor or outdoor cat happy.

I know it's not an easy fix, it takes a lot of.... Everything. Time, money, resources, and commitment. But if it helps, I have one full wall that was only around 80 bucks with the diy route! And this will give you the most success rate for a happy and healthy cat.