How to know if a cat is experiencing a partial/full urinary blockage?

I recently adopted my cat a month ago. He is a DSH, part Russian Blue. He is 1 & 1/2 years old. 11 pounds (good weight since he is a long cat).

Since a week ago I have noticed that he was not running to his food as enthusiastically as before. He would either ignore the food or smell the food and go away & eat it at his own pace. 

Then I would hear him meow occasionally in or outside the litter box (just one meow each time), he would stand there and leave within a few seconds without using the litter box. Then later on, he would come back and use the litter box with no problem from what I could see. 

I took him to the vet, they took a urine sample and found crystals. They gave me gabapentin and it has been two full days where I have given it to him 2x a day. He still occasionally meows at the litter box and not use it. But he will then pee at a later time (I attached a video of him using the litter box). 

How do I know if this is a blockage if the only symptoms he has is not enthusiastically eating his food and going in and out of his litter box. Just to let you know, he pees about 2-3 times a day and poops 1x a day. 


I really cant afford an x-ray right now so I really want to make sure if I need to go the emergency now.