(1) The if here means when(ever). As Richard suggests, the sentence refers to something habitual.
(2) with such if-clauses, the main clause can take either the modal will plus verb or a simple present verb. If a past habitual event is involved, it takes would plus verb or a simple past verb. Other tenses are also possible, depending on the intended meaning of such a sentence. What is most important is that the will in such sentences seems to be semantically different from that in the simple future sense as in the following example: If it stops raining tomorrow morning, I will go to town.
(3) The question in full reads either (a) or (b):
(a) Whenever he is not treated with the respect he feels due to him, he gets ill-tempered and grumbles all the time.
(b) Whenever he is not being treated with the respect he feels due to him, he gets ill-tempered and grumbles all the time.
(4) One further question: can being treated mean the same as treated?