I get the impression you're rather set on the idea that this pig needs to be out and about in the City - which isn't realistic. Guinea pigs are prey animals. They are afraid most of the time. They aren't like dogs, who enjoy being outside interacting with new people and animals. It would be much better to make an enclosure, perhaps with extra grids, and set up a safe play area outside on your lawn or a safe place on your apartment/condo grounds. It's nothing difficult and a great deal safer and more enjoyable for a guinea pig than trying to put them on a leash and let them interact with the world like a dog would.
Also - citing a veterinary program as a means to bolster your argument isn't necessary and really not at all effective. I'm sure if you searched long and hard enough, you could find a vet who would recommend feeding Chex cereal and low fat dog food to your guinea pig, too.
There is no doubt you love your pig and no doubt you have a wonderful little guy - which I think means you'd love to hear information that can help you be a better caregiver for him and provide him the safest possible life. Which is what this forum represents and hopes to help owners be, by providing information and educating them on dangers they may not be aware of otherwise. No one means to attack you or make you feel the need to be defensive - just let you know that there's a better way to do things. Which, I suspect, is probably why you joined because you obviously love and care about your pet and want the best for him. Right?