Hi there - I'm Dr. Sara. I'm a licensed vet who works exclusively with dogs and cats. I'm sorry to hear that Bahgeera isn't feeling so well - I have some thoughts that you should find helpful.
On the subject of the tooth, when you are seeing that "dot" in the middle of the fractured tooth, this is a sign that the root canal has been exposed and this can cause significant pain. The discoloration usually indicates that the vital pulp has died and could potentially be infected. It wouldn't surprise me at all if he were reluctant to eat due to discomfort associated with that tooth. Unfortunately in my practice, when we see a fractured tooth like that our first recommendation is for extraction. A board certified veterinary dentist can often do a root canal +/- cap procedure, however these are significantly more costly and can have a finite life span.
So I would suspect the tooth first, but in the interest of not having tunnel vision, when I have a patient like this I usually check a fecal sample for intestinal parasites and a blood panel to make sure there's nothing else going on.
My own dog was recently showing signs of decreased appetite (not severe, but noticeable) and began to lose weight. In a panic, I ran full blood work, but did not find a cause. He had accumulated a good bit of tartar on his teeth, so I scheduled him to have his teeth cleaned. Within a week after his cleaning, his appetite was back in full swing like I hadn't seen it in six months before. This experience of my own really cemented the idea that dental discomfort can absolutely affect their appetite and quality of life.
I would suggest scheduling a consultation with your vet to discuss his tooth and check to be sure he's in good health otherwise.
Please let me know if I can answer any other questions for you!