Welcome to Yorkie Talk and congratulations on your new litter. It sounds like you are doing the right thing by making him attach. Do you get up during the night also to make sure he is nursing? It could be that even though he appears to be nursing, he might not be nursing long enough or strong enough to get enough nutrition. You might need to also supplement him with a bottle or dropper. Do you have Nutri-drops available over there? They work good for a weak puppy.
Here is an interesting article that might also be useful. I have used the glucose solution they mention on small puppies and I really felt that it helped.
http://www.showpapillons.com/formula.htm
It is also possible the puppy could have a small hole in the roof of its mouth (cleft palate) that prevents it from sucking well.
I do wish you the best with your new litter and I pray they all do well.