Eastjen, Thanks a lot for your help, I am currently working with on sit or down/stay command under this kind of situation.
As for training collar, I already have a choke chain, which I do not like. Even the plain collar can make him uncomfortable when he is pulling hard. I am considering the easy walk harness.
Another episode of small dog encounter, it was actually happened in my home, we had a guest with a 3 years old Maltese. He first found out his pee pad and smelt it carefully, and then stole a piece of treat from the dog carry bag. He was shown to the dog later. He was so happy to see him that he rushed to him with tail wagging vigorously and began sniffing all over him. The Maltese was not happy, and tried to avoid him, but he still kind of all over the little paw, we had to drag him, he was so sad that he to whine and bark. Then I led him outside to cool him down. After a while, I led him inside, and let him down/stay 5 feet away from the little paw on the sofa. He still tried to jump and I was able to make him only look (longingly) at the small paw. When we put the saw paw on the ground, he exploded with jumping, loud whining and barking again... After the guest left, he was so exhausted that he drank some water and laid on the carpet... It's pity I didn't have chance to take pictures amidst the chaos.