I got one for our old mastiff boy who weighed about 70kg on Amazon, I think it was by Doghealth? It gave him a complete new lease of life as he could finally get in and out of the car without it hurting him. It took a bit of persuasion to get him to use it in the beginning but after a few days he loved it. He didn't always wait for it to be folded out for him to get out though bless him!
Try putting the ramp on the floor, perhaps in the house, let him get used to it. Then try gradually increasing the height. Use high value treats - freshly cooked chicken perhaps. Slowly slowly ... try not to get stressed. I feel for you, hope you sort it soon.
Orvis. We had a standard tri fold one, but new car was higher so needed longer ramp. Not cheap but we got this one http://www.orvis.co.uk/p/natural-step-dog-ramp/161m DDog happy with it until November last year, when legs just couldn't cope any more. ((DDog is 14.5 and around 45kg). This ramp v light and doesn't take up much space.