How do I calm down an overwhelmed cat?

I adopted a healthy, already de-clawed cat from the animal shelter two years ago. I take him to the vet every six months for a check up. He has all his shots and is very affectionate and mild tempered. Since he was already de-clawed, my vet told me that I should keep him indoors because he would not be able to defend himself against another cat or a dog without claws. However, I became saddened by how my cat would stare out windows and meow at the door like a dog wanting to taken for a walk. I did some research online and found that I could put my cat in a harness and take him outside. I got him fitted for one and let him get used to it while indoors. However, when we finally went outside (some 3 weeks later; he kept flopping over when he felt the harness on him) it looked like he had a sensory overload and was all over the place. He rolled around in the sand which got him all tangled up in the leash and hissed at me when I picked him up. I've literally only heard this cat hiss twice in the two years I've had him; once at a dog in the vet's office and the situation I just described. I'm unsure of what to do now. How can I calm down my cat while outside? I want him to enjoy the outdoors safely. Our next vet appointment is two months away.