Temporary paralysis in kitten happened over and over

I adopted a 2.5-month kitten about 1 month ago. A few days ago, when he was playing with a nylon bag, I suddenly picked it up, he was surprised and scared, he became agressive. I thought he was just mad at me, but he kept lying for a few minutes there and meowing painfully, turned out he couldn't move his rear leg. 10mins later, he could use his left rear leg but his right leg limped, still meowed painfully. I took him to the nearest vet the next day and they said it was just trauma and injected him with medicine. He recovered pretty well and could use his rear legs through it seemed weak.
But about 4 days later, he was scared by the neighbor's dog, went paralysis again, he sounded very panic and painful, 30mins later, he was fine, got back in my house.
And today, just afew hours ago, I accidentally scared him, he became like that again. I'm afraid that he might have serious health problem, is anyone know what could cause this?

Sounds more neurological than physical personally. Did your vet take x-rays to confirm there was no serious damage to the bones from the trauma? If it is physical it could be that when startled he is moving the wrong way and hurting his injury which could indicate something worse than a bruise or bump. I am not a vet nor do I claim to be but I would go to a different vet and get a second opinion. 

Just to note, on the dog, was the kitten outside on his own? A young kitten should always be supervised when outside. Hawks, owls, raccoons and other predators will scoop up a kitten without hesitation and kittens that young are usually having too much fun to pay attention. I do indoor/outdoor so I don't have an argument on letting him out but mine are always supervised outdoor play time until 6 months and get trained on coming when called during that time. 

Sounds similar to those fainting goats that fall over when they get scared.

He might be having TIAs (transient ischemic attacks).  These are precursors to strokes, like mini-strokes.  But they don't necessarily have to result in a stroke and can be treated medically. 

I'd take him back to your vet and mention this possibility to get it ruled out.  Then, you can focus on possible psychological triggers from PTSD (post traumatic stress disorder), which can be easily treated with CBT (cognitive behaviour therapy).

Both these suggestions are treatable.  Try not to worry but do take him back to the vet  

Please excuse my medical jargon but they will be helpful when you talk to your vet.