At his age, and if he is getting lethargic, you may need to use your finger to scoop some canned food in to the front of his mouth- just a finger nail size at a time. Once it's in his mouth, he should reflexively chew and swallow.
Do you have nutrical on hand? If not, a small dollop of Karo syrup will work in a pinch to help bring up his sugars. Just make sure you follow that up with some protein such as canned food or slivers of cooked chicken, otherwise his sugars could rebound and drop even farther.
Once you've gotten a little bit of food in to him, let him rest a bit. 8 weeks is young and he's had a very busy day. It's quite normal for the ones this small to sleep 18 hours or more a day (though not in one sitting!) Give him an hour or so to rest, and then offer more food again.
Above all, breathe. These little ones are scary to us first timers. Become familiar with the signs of hypoglycemia so that you can act quickly if need be. Keeping him warm will also help reduce stress (his and yours).
Hypoglycemia often seen in tiny or underfed yorkie puppies and Yorkshire Terrier adults