I would never have dreamt of confining any cat to either the garden or indoors up till about 10 yrs ago, when I lost too many beloved cats to RTAs. I seriously thought about not having cats at all because I couldnt bear the thought of losing anymore, but equally I couldnt bear the thought of not having a cat (or two LOL! ) around. So I decided firstly on a large cat run, which all but one (Harry) adapted too. Although it was large, full of plants, grassy areas, trees, shrubs, hidey holes, high shelves - I was never completely comfortable with confining them.
Then I moved to where I am now, the cat run came too and was re-assembled, but over the years, because I have quite a large garden, bordered by other gardens in a small cul-de-sac, away from any main roads, I gradually started letting them out into the garden, while I was out there, and started to feel more comfortable about letting them have more freedom, whilst still having the security of the cat run, when I wasnt around or at night time. Now, I have a couple of areas that are cat-proofed fenced which they can have access to at any time and which I can close off from the main garden at night time, the rest of the time they have access to the whole garden, which is where they stay (apart from Harry and he goes out whenever/wherever he wants LOL!) It is a lovely feeling now to know they are secure, but that they also have that freedom of the garden when I'm around - and really, they want to be where I am, so if I'm out in the garden, they are there with me.
If I were you I'd probably try cat proofing the garden for him first - if he adapted to that idea and seemed happy that would be brilliant wouldnt it?