Yes the stress and fluids loss and the energy requirement for healing can cause the body to consume stored calories to cope qhich would result in the weight loss. His stomach may not be processing food quite as well as usual while it heals too.
See how he is next week and talk to your vet about it.
You can read about bloat here
www.marvistavet.com/html/body_bloat.html
And you did a great job identifying it and getting your dog to care in time!
Hope this helps you!
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