Toy Xolo

Question
QUESTION: I have three 6 month old toy xoloitzcuintles (2 males, 1 female) and a 3 year old female.
When the puppies were younger none of them used to cry at night, but a few days ago one of them, a male named Tiku, started crying alot! I feed him and stop crying for a while, then he started to cry again, so I gave him water, after a few minutes he started again, and I took him out to pee, few minutes later the same thing!
I decided to let him cry, but he cried for hours!! he started crying at 12 am and stopped at 8 am!!!
He is kind of shy with other people and with animals, and he barks a lot when I take him for walks, specially when he see a dog... When I'm with the 4 of them, he wants to be close to me and when his siblings get near me he barks and try to bite them.
What can I do to make him more confident and to help him to be less aggressive with his siblings?
He is currently being trained by me to be shown at a conformation show and is doing great, he is just aggressive when it's about me or food.

ANSWER: "Showing" any dog of any breed is fundamentally about type and temperament.  That dog is not temperamentally suitable for breeding.  If you're running him on for the show ring, you're not doing your breed any favors.  If he's "shy" with other people and animals, how do you expect him to behave appropriately in the breed ring???? Have him neutered and put him in a pet home.

You are inadvertently training his night time crying behavior by consistently responding to it.  It will take quite a while, and a lot of determination from you, to extinguish this behavior...ONE NIGHT will NOT do it.  Any trained behavior in any dog can be subject to response perseverance, which means if the dog has been heavily rewarded for a long enough time (and especially from a tender age), the learned response will be much more difficult to extinguish.

Regarding his aggression toward his litter mates, I do not know how well these puppies have been monitored since being whelped.  This particular dog may very well have been last at the teat, pushed around by his litter mates, or (conversely) first at the teat and bullying toward his littermates, all of which should have been closely monitored by you, the breeder.  He may be lacking confidence or he may be suffering from an overabundance of confidence.  I have no way of knowing this from your post.  Please provide further information.

---------- FOLLOW-UP ----------

QUESTION: Thanks for answering so fast...
I took this puppy to several shows before so I could see how he reacts, and he did amazing; he let people touch him and was friendly with other dogs... he was very nice with his littermates and there was no problem with him.
But a few weeks ago he started acting like this; he is demandings alot of attention specially from me.
He was always last at teat and pushed around by his little brother.

Answer
Being last at the teat is definitely a problem for him.  His pushiness (by demanding attention from you) is a struggle for rank with his littermates.  Use positive reinforcement training (but don't teach SIT since this becomes an automatic response and is a huge no no in the breed ring) on ALL these puppies to maintain psychological dominance, but especially on this one.  The Xolo can be a handful and nothing worse than seeing a good puppy going wrong.  Learn about positive reinforcement training by going to karen Pryor's web site; you'd also do well to read Patricia McConnell Ph.d.'s work on multiple dog households and problem behaviors.  Instead of offering this pup free attention, make him work for it.  This should keep him in line psychologically but will also keep him comforted in terms of acceptance.