My dog (Leo) is a 3year old chihuahua.He has been in the family for about 9-10 months. Until here recently he has been a wonderful dog to own, never growled, snapped, or acted any kind of weird towards anyone.
I recently moved out of town and in with my mom and her two dog; I also own another chihuahua and he's about 5 months old. Her dog just went into heat and they have been tied up 4 times now. When we put them in different rooms, he sits by the door and wines and begs. I noticed he's not himself at all. Won't play or pay any attention to anything else.
I have a 3 year old son and Leo is just getting to aggressive he have him around. Hes bit me, my husband, and my sister already. Sometimes he won't let us past the door without barking, growling, and biting us...
Please, what do I do??

So you're trying to breed this dog? Why? Are you a registered breeder? If so, how many shows has Leo won? Is he registered? Have you done genetic testing on him?

You put him in a house with a female in heat, he's obviously not neutered since you're letting them mate, and you wonder why he's aggressive?

If you want to breed, then breed, and do it right. If you want a pet, get him fixed. You can't have both a breeding male, only used for breeding, and a docile pet.

He's aggressive because of the situation you've put him in. If you knew anything about dogs at all, you'd realize that.

I also am a bit stunned at your cluelessness of your dogs behavior, things you should be aware of if you own a pet.
That being said, let's break this down a little.
1-He is intact (not neutered) and he has been put in the position of being at he mercy of his NATURAL instincts.
2-The hierarchy has been challenged, not only for your dog but the others who he is now sharing space with.
3-He is in a new place with new family (people family and dog family)

I can only imagine the frustration he feels, his lashing out, as I can tell is completely legit.

No, that does not mean he gets to bite people, my suggestion would be to have them all spayed/neutered and speak with your vet about behavior modification that fits his situation, his actions and reactions to all the changes he has experienced.
And why are you guys allowing them to tie, keep them separated, there are enough unwanted puppies in the world, shelters are full of them, the streets are full and it is just sad.
As far as your child goes, your job is to protect him also, you do whatever it takes to keep him from getting bitten. He is at the tail and ear pulling age and can completely pi$$ off an already frustrated dog or any dog for that matter.
Neutering alone will not be enough, it will help, hence the behavior modification.
But be prepared to do a lot of work with him and the ENTIRE family will have to be on board with this, if you can't commit to that then you may want to consider re-homing him.