How do I calm my dog down before he is euthanized?

I have an 11 year old English Setter who was diagnosed with a brain tumor in February. He was doing really well until recently. We had him on 200mg of vetprofen and that suddenly seemed to stop working. Our vet told us that increasing the dosage or changing the medication would be our choice, but she recommended that it would be best if we stopped after 200mg. That's what we're doing; we would rather let him go on a high note rather than hit him up with tons of pills until he goes. Our vet also gave us some percocet, but she said only to give it to him if the vetprofen stops working and only for a maximum for 5 days (it's just to keep him out of pain until we can get him in for the euthanasia.) We started him on the percocet Friday and we're taking him in on Monday. It seems to be working well but he's still walking crooked and falling due to his tumor. He's still wagging and kissing everyone like his normal self, only he's weak and sickly. He's a good dog, but he's an anxious, nervous wreck when getting in the car, going to the vet, and being at the vet. Is there anything I can do to calm him for the ride and everything? The last thing I want is for him to be stressed...