I have grown to grudgingly accept that cats will be cats.
As your cat has been used to freely wandering outside in the countryside, I think you should be considering this fact the most.
He has experienced the outdoors, and knows the existence of 'outside'. I don't think it is really possible to undo this. The cat may forever yearn for what is out there. They are explorers, after all. You may adversely affect your outdoor cat's mental health by restricting him to inside-only, and he will likely take advantage of any opportunity to escape.
My subjective opinion here is that it borders on cruel to deny an outdoor cat access to the wider world. Especially if is he is an 'Entire Tom', with balls intact.
In a real urban area it is very likely to be in you and your cat's best interests to restrict, even deny, his access to outside. As a responsible pet owner you must of course do what you think is best for your cat.
Neutering will be essential. If he is not castrated, he will roam, and he will get into trouble! He may disappear for weeks at a time, and come back for a meal before disappearing again...
A neutered cat is not driven to roam, and is much less likely to get himself into trouble. Although he will look at you angrily from time to time.
In my case, I exist 'in partnership' with my cat. I provide him love, food and shelter, in the same way I provided his mother and grandmother love, food and shelter. In return he provides me with scratches, hairballs and cat-poop.
He comes and goes as he pleases, and if I lose him to the wider world I will be distraught, but I grudgingly accept this as a fact of life.
Cats are cats! This fundamental fact really means he should have access to outside.
Though if you choose to keep him indoors, please do one thing. Get a plant pot and sow some grass-seed and keep it by the window. He will need to chew on a blade of grass once in a while, to help with an upset stomach, especially if he is a long-haired cat.
Absolutely have him micro-chipped, and don't bother with a collar. If you must, use a safety-collar designed to break away under pressure. By using one of these collars you more-or-less guarantee that your cat will soon return without a collar.
All the best to you and your cat.