I have a 6week and 4day old chihuahua and he seems depressed. He eats and drinks. He does not play much he is nothing like his little sister who was the runt. Mine was the second born. Is this normal and what can I do?

Is this pup away from mom and littermates or do you have the whole clan?
6 weeks is actually a bit young to be away from mom if that is the case, they need to be with the litter until at least 8 weeks old if not a little longer, quite a bit of their socialization happens during this time.
Is there anything else going on with your pup, any vomiting or diarrhea?
Look at the whole picture, if your pup is eating well, not vomiting, having no diarrhea and is alert with interaction, then I would think that you just have a puppy that likes to sleep, which is normal.
Just a note, it is important that puppies have the proper vaccines and be dewormed. By now they should be have been dewormed at least twice if not three times.

Don't want to freak you out, but I just witnessed a whole month of "depressed" puppies who are eating and drinking, coming in to the hospital and testing positive for parvo.

Any abnormal behaviors in such young, small breed puppies should be checked over by a vet. Parvo is a nasty virus that you don't want to deal with.

Also, small breed dogs are prone to getting dangerously low blood sugar levels. Make sure your puppy truly is eating many small meals throughout the day!

Hope you just have a low key pup!