Keeping my rabbit outside, inside, or both?

I have a 5 year old, 3 pound Holland Lop rabbit. He has lived outside all his life. I understand there are many risks of rabbits living outside, but considering my situation, I believe I had alleviated many of those risks.

Predators- My rabbits cage is inside of a larger cage, that you can walk into (used to be an outdoor iguana cage). This makes it hard for a predator to get near the cage and the larger cage has a secure lock that locks behind you even if you put physically lock it. I understand rabbits can die just from the presence of a predator, but my rabbit is fairly calm and doesn't really know that predators are bad, so he just isn't as scared of him. Also, I have very few predators in my neighborhood.

Theft or teasing by humans- well, there really are no people in my area that would try to steal a rabbit.

Moldy or poisonous plants- my rabbit does not have access to the ground/plants

Toxic pesticides or fertilizers- we do not use those in our yard/garden

Exposure to sun, heat, wind, or wet- His cage is in an area that is fully shady all day in the summer, it does not get too hot where I live, and when it does we keep a very close eye on him and provide him with ways to cool down, sometimes we take him inside for a little bit. He has never gotten too hot or cold. In the winter the big cage is covered in plastic to protect form rain/wind but one side is left open for ventilation. This makes it so he does not get wet or hit by wind. The cage also has a roof

Bacteria contained in dirt- I bleach the dirt on the bottom of the big cage often, plus he is not allowed to play in it.

Diseases spread by flies and mosquitoes- we do not have mosquitoes or biting flies in my area

So, he has so far been totally healthy and never had a health problem. He is very happy and has a very large wire cage (10.5 square feet), he also gets to run around in a 36 square foot dog kennel in the grass, that has a roof and wire on the bottom. He gets to be in for a few hours at least a day.

So, my only concern is that he is getting older, and will probably be more susceptible to disease. My parents will not allow him to be in the house, but they might let me keep him in the basement. Our basement is a nice and cool in summer, but not too cold. I have a cage I could keep him in, but it is only 24 inches by 36 inches. I would still be able to let him out in the pen during some of the day, but not all the time. He doesn't really like being inside and is unhappy when he is in a smaller cage. He seems pretty happy outside, I am not sure what to do though, suggestions?input?