Don't worry about it too much, he'll get used to it. My cat was a stray, and is now indoor-only, and she is very, very happy. I take her out on a leash every once in a while, and she loves to dig in the dirt and crawl around the bushes and get burrs and stickers all up in her fluffy coat. >:( I got her this harness and it was hilarious when I put it on her for the first time--she would only walk BACKWARDS. I only put it on for about a minute and took it off right away once she started walking backwards, but it was so so funny. It took quite a few short sessions over a couple weeks (with lots of treats!) to get her used to wearing it, and then it took another week or so to get her comfortable wearing the harness with the leash attached too. And then weeks and weeks of taking her out in the front yard gradually (starting with just sitting on the doorstep) for her to be ok with it. Now she loves digging in the dirt and rolling around on the lawn, but is still SUPER spooked when a car drives by, or a person comes by on a walk, and NOPE NOPE NOPE to dogs.
I take her out every once in a while, but honestly, she's really just happiest inside. I made inside waaaaay more appealing than outside to my cat with lots of attention, training, treats, toys, and a fun environment, so outside no longer holds any appeal. She likes to look out the windows and watch the birds, but actually going out there to chase them? Nah. She's happy with crickets and moths and spiders that show up from time to time in the house. And although she really does love digging holes in the dirt, she'll only do it for about 10 minutes or so, and then she is DONE and runs to the door to go back inside.
I think the hardest part for you will be getting him used to the harness, but since he loves the outside already, that part will be fine, and I'm sure he'll appreciate it!