He is only eight weeks old, I presume you have not had him for more than a couple of weeks?
He is only a baby and in the past half of his life there have been a lot of changes. His world changed totally when you got him, his litter mates all disappeared, his familiar surroundings went, his food bowl and water bowl changed and he is totally dependant upon two complete strangers! He is in a massive house (how would you feel if someone kidnapped you, did not say a word to you and left you in a castle all alone) and he does not know where anything is or even if you intend to kill him or not!!
I would be scared!!
His mum used to show him everything, what to be scared of and what to play with, what to eat etc. Now she has gone and he is left to his own judgement!
He will grow out of this phase as he gets a little older and gains confidence in himself and gets to know you both and feels that not only can he trust you but he can love you too. Soon he will become the big dog in a tiny dogs body that chihuahua's normally are!
Don't worry, he will grow out of this phase, just be consistent, giving him his own little 'den' where his bed is might help (something like an indoor kennel with a blanket draped over it and the door left open) somewhere he can call his own that he can come and go when he wants to, rather than having to be in a massive (to him) room.
Good luck, enjoy him while he is still a pup!!