Oh my gosh, your poor baby
! I don't think it's neuro or PTSD, personally. He is in pain and straining, like you said. There is something really wrong here and I think I'd get him to GI specialist asap.
It could be structural in the rectum, it could the anal glands - are the glands swollen/protruding?
Does the more liquid discharge smell "fishy"?
Has fiber been tried?
What is the diet?
If he has almost constant anal leakage, he probably does need a specialist rather than a regular vet.
Also, you can use diaper cream/hydorcortisone cream on his bottom to soothe it. The reason he acts scared and runs to his bed after pooing is bc he is probably still in pain, he just expended an enormous amount of energy, and the whole process is very scary for him. After my dogs experience any pain, they are scared, tired and want to be held by me.
I feel so badly for Thor!