I think he is very cute. I think the t word is used to warn the potentual furever family of possible problems with small dogs not as a way to get more money etc. To a group that has larger dogs a possible 5 lbs seams almost unreal tiny.
Because it is a rescue I would say go for it.. can you go in person? I would be careful about shipping and such (I am sure you know that) Also .. know like the others said .. this wonderful little guy could gown to look more like a pug or a chi then a yorkie and be a bit bigger..but he will have a heart and personality to love
