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Most important thing is that everyone in the household signs up for NO TREATS between meals. Nature Diet suggestion is good. I'd definitely endorse getting him seen by the vet, and would suggest short frequent walks. If he's come straight from the previous home, you will almost certainly need to make sure his worming is up to date.
I'd also get into doing short bursts of clicker training with him each day - will help hte bonding process, keep him on his toes and can be a way of him getting some of his daily food. There's a Chi in my current dog training class who has taken to clicker work very quickly.
We have a similar foster greyhound bitch who only has four teeth - we mainly feed her on the frozen blocks and mince of raw that you get at PAH - by AMP, with mixed in rice/veggies/occasional oily fish etc. Once the dental work has healed you will find that the gums toughen up and our girl will tackle most things up to and including a pigs ear -she sort of sucks it to death. She will also pinch kibble from the others' bowls if she fancies it - pretty much the only thing she won't tackle is a Bonio or a chicken's foot.
Feed nature diet? Nutritious, palatable (except for the fish one) and it is a wet food in pouches so no teeth issues. One pouch would probably do you a few days! ( not in touch with quantities for minis as mine are at the other end of the scale!)
We completely out of the blue have ended up with a 6 year old chihuahua who is twice the weight he should be. He has been beaten badly in the past and has no front teeth apart from this four canines. Due to his weight his breathing is very noisy and he struggles to walk.
We only got him this afternoon. How do we go about him losing weight? He has never eaten dog food before, always had scraps/human food. We are putting a stop to this immediately.