He is making loud noises when trying to make a bowel movement and he is shivering?

It could be a blockage - take him to the Vet - NOW!

Can he poop? Or is he straining?

If he is pooping.. I would say anal glands are impacted. Its very painful, and you can tell by looking at their bucky. The anal sacs will be very swollen and can be felt on top of the skin. You would need to go to the vet for them to be expressed.

If he is straining, then probably a blockage.. That would be an emergency run to the vet