My brother in law lost weight suddenly and his teeth loosens and falls but never bleed. Gets tired very fast.?

Go to doctor asap. I think your brother-in-law should see his physician as soon as possible. Sudden weight loss, fatigue, and tooth loosening is a definite sign that some kind of systemic illness is going on. All kinds of diseases show signs and symptoms in the oral cavity.
See MD ASAP. Rapid onset of weight loss and fatigue are 2 symptoms that warrant a full check up. They could be the result of anything, mostly not worrisome, but can be an indication of something serious too (do not delay seeing an md). Good luck to him.