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Hello, this is Dr. Bruce and I would like to help you with Buddy.
I'm sorry to hear that he is having this issue.
I'm a little concerned about the foaming around the mouth. It makes me wonder if he has been exposed to some sort of toxin, or if he might even have some sort of trauma to the inside of his mouth (from chewing on something, for instance).
You don't mention whether or not he has received all the appropriate puppy vaccines, or if he is on a heartworm preventative. If not, then I would be concerned about potential infectious agents such as viral diseases, or parasites.
The behaviorial changes are also a little disconcerting. It could be just because he is feeling off, but it may also indicate the possibility of toxic or infectious problems.
I would strongly suggest taking him to see his vet tomorrow for evaluation. If I were to suggest a course of action, I would recommend checking him for parasites (fecal examination), check him for a fever, look in his mouth for any trauma or foreign body, and possible bloodwork if nothing else is obviously wrong.
I hope this helps and that Buddy is doing better very soon. I have dachshunds myself, so I imagine that he is a particularly cute combination of low rider dogs! If you would like to continue this chat, please reply to this post and I will get back to you as soon as possible.