Seriously though, get him a harness and a lead that buckles into the seat belt. He’ll thank you for it if you’re ever in an accident, god forbid.
Awesome look pup. You did good by rescuing him.
Do you live in the US?
I don't, but have gotten the impression that unsecured dogs is the norm there.
Anyway, you use something like this. Or a nice crate.
I've heard somewhere that a crate is safer, but I've got no idea why. We use a crate which we've secured with the seatbelt running through it. He's very calm during rides and being left in the car alone.
It’s absurd to drive without strapping in a dog. Not only is their life in danger, but so is every passenger’s in the car.
https://www.amazon.com/dp/B01KNULPO6/ref=cm_sw_r_cp_api_i_7ImTCb4QDR9GE
You can buy a harness with a buckle to keep doggie safe
https://www.amazon.com/dp/B01KNULPO6/ref=cm_sw_r_cp_apa_fabc_83K0D8ECZN7RN2K612QC
Pawaboo Dog Safety Vest Harness, Pet Dog Adjustable Car Safety Mesh Harness Travel Strap Vest with Car Seat Belt Lead Clip, Suitable for 11 lb-33 lb Dogs, BLACK https://www.amazon.com/dp/B01KNULPO6/ref=cm_sw_r_cp_api_cBU3AbABDTKQ6
One thing I would consider is that a single restraint point is not going to save your dogs if you have an accident, especially if it's just attached to a collar, in fact it would probably kill them. I would look at a harness system, something that would spread any pressure from an accident out over their body, like our seat belts do.
Or a three point system like http://sleepypod.com/clickit