Excessive weight loss

Given the very limited information it is impossible to say. And even with plenty of information it is nearly impossible to say. 

Another question is, Does knowing make any difference?

Well one practical answer to that, is that if a doctor states his life expectancy is less than six months he may be entitled to certain state benefits. 

Another answer is funeral planning, and writing a will, but that should be done irrespective of life expectancy.