Welcome to YT, so glad that you found us and thank you for rescuing this sweet little boy. You didn't mention he's weight, the reason I say this is because my now passed girl weighed in at 17 pounds as an adult, she had the longer snout / muzzle that your Chewie has. My rescue boy weighs 12 pounds and also has the longer muzzle. The bigger the yorkie the longer the muzzle is what I have found with my yorkies, all of mine were 9 pounds and over, the 9 pounder had a shorter muzzle, all were AKC registered. Your Chewie looks full yorkie to me from these pics but, I have seen Morkies that look pure yorkies so it's hard to tell if he is in fact a mix, you could do a
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As a rescue it's very challenging because we do not know their past history, abused, caged, not socialized, everything is trail and error.
I have to agree with holding him when your guests arrive, not a good thing, makes him feel like you are protecting him from harm. I agree with lovetodream, let your guests come in and just ignore him. I had a baby raised by me from a 8 week old puppy that was scared of men, I believe because my family consisted of only women and a mans deep voice scared them, she would nip the back of their shoes. There were also a few woman that she didn't cozy up to right away. I would tell new ppl just ignore her, let her come to you and sniff you, once she has your scent you'll be her BFF. I would give them a treat to give her once she sniffed them and walked away.
Having your guest sit on the floor and having Chewie go to them is very good you should give your guest a treat to offer him in their open palm, let Chewie take the treat.
Teaching Chewie basic commands is very important and builds confidence in the dog while building a strong bond with you. Sit, stay, is a good start so you don't have to hold him to let ppl enter your home. Short training periods of 5 to 7 minutes several times throughout the day with high value treats and happy hand clapping praise. If he is food motivated and loves his kibble you can use that, many dogs love cheerios, just one tiny cheerio or kibble when he does what you ask. Train only one command at a time, sit, once he has that down pat move on to stay, once he has that down pat put the two together, sit, stay and make him stay in that position for a few seconds, when he does, treat and happy voice praise, reach time make him sit and stay longer, say a minute, if he holds that position, the next time make it longer, say 2 minutes, always treat and happy voice praise. Your goal is to have him sit and stay on command when you get company so he doesn't dash at them when they enter. Lots of patience and consistency are key factors in training commands. Keep it brief, several times throughout the day.
Your post does not indicate you are complaining against your baby, it shows you are committed to making this little boy as happy and stress free as possible. He is a lucky boy that it was you that found him as you are committed to his well being.
As for outside walks you may need a personal trainer for that. There are many things, ppl, sounds, smells that can scare a dog that has not had proper socialization as a puppy.
The toilet brush, before you move to the next bathroom put the brush in a plastic bag so he doesn't see it, hide it lol, then leave that bathroom and move on to the next one, always hide the brush.
I hope some of this helps you. Rescues come with a lot of baggage that we will never know, it is up to us to try to rid them of their fears and gently as we can, it can take a long time so patience is a must.