It would be a good idea to have your Chihuahua's teeth checked out by your vet and make sure that there isn't a problem. Having said that, let me ask you this. Have you ever watched a child eat candy from a bowl where there is hard and soft mixed together? The soft is easier to eat and more of it can be eaten. Dogs do think logically, why eat the hard food when you can eat the soft and eat more of it.
The main thing you can do for him is take him to the vet to have his teeth checked. Chihuahuas sometimes have dental issues and even though he's young, it's not uncommon for them to have dental problems at this age. It would be a good idea to have it checked now so that if it IS a dental problem it can be nipped in the bud before his teeth all start to go bad and an infection spreads through his system. It may be something simple like one bad tooth or it could be the start of lifelong dental problems, but you won't know until you take him to the vet. From the symptoms you gave, this is about the only thing I can think it could be right at the moment. Please let us know what the vet says.
Ghilly