I think he needs both calcium and magnesium. The calcium causes the brown spots and the leaf edge necrosis. The magnesium causes the interveinal chlorosis. At least, that's how I've always read it (the leaves, that is).
Where is the worst damage, at the top or bottom of the plant?
Good question, and good call on the question.
Based on the size of the leaves, the red petioles, interveinal chlorosis along with the spotting I think you have a combination uptake and deficiency problem. This is without knowing what you've been feeding, how hot or how frequently.
Foliar with a good
Ca/Mg supplement, should help all those issues. The damaged leaves will not regenerate, but your goal is to prevent further deterioration which is a sign that the plant is using up stored reserves and is running out of this extremely vital elements. That plays into other things because all of these minerals/elements have reactions that are what comprises life processes.