Julio is adorable! My brother and SIL have a chihuahua who travels with them, and Paco has really won me over to the breed. They are such bold little characters. He is quiet and crate trained and will retreat into his crate voluntarily to find a haven if there are too many people around or he's just tired. It's his "den", and dogs are just naturally den animals, as long as you never use the crate as punishment.
When he gets past the teething stage you could make a quilted "crate" with a zipper and mesh windows. You can get a special mesh in fabric stores. There are patterns, or you could design your own. The crate can't be too big because the idea is that a dog will not soil its bed. To tether a crate in the car, pet stores sell tethers that anchor to a metal bar under the seat and then thread up under the seat back. That would usually work better than the seat belt which is designed to reel out when pulled slowly.
Several people have mentioned the dangers of falls, but it can't be stressed enough. A friend of mine had a tiny poodle that jumped off a groomer's table and shattered her leg so badly it had to be amputated.
Wishing you many happy years with precious Julio.