Twitching and Constipation

I am hoping someone can help me kitty.

I just adopted him from the shelter and he has some mild issues, which the vet thinks might be from vaccines and/or flea control drugs. She says that the flea control medications can cause "nervous system issues" in some cats.

He twitches/spasms a lot in his limbs when he lays down to rest.

He is constipated. He has a bowel movement most days, but it is hard and dry and comes out in lots of small chunks. Sometimes he has a bowel movement and goes back 20 minutes later to finish it.

He gets irritable sometimes and seems uncomfortable with random sensations in his body. He doesn't have any fleas, but he jumps at certain parts of his body like he does, then he is very irritable for a little bit after - it seems like random pain or more like random sensations in his nervous system.

He is generally precocious, smart, very active, a bit on the restless side, but his restlessness has calmed down some now that he is settling into his new home. He is social and follows me and my husband around - likes to be in my lap and likes to play with my husband.

He is not very interested in food - but eats enough to sustain himself. He is thin, but healthy - not too thin. We are switching him to a healthier diet. He is about five years old.

Oh, his only fear seems to be doors. He gets scared when he hears the doors to the outside open. I think he must have been kept indoors his whole life.

I was reading about Thuja and Silicea for vaccines and both seem to cover constipation too. He doesn't seem shy to me (Silicea), but I am not sure of the temperment of Thuja?

Thank you for any help.