The problem with your question is that there is not very good evidence to answer the more basic question: what language was spoken in "Macedonia" in the late 5th and early 4th centuries BCE. Most scholars agree that it was some form of Greek, but which dialect is not entirely clear (a variant of the NW Doric, or a "Hellenic" cousin language of Greek, for example). I'm not well-versed enough to parse out these different theories here. In general, though, the grammatical structures, the basic vocabulary, and the underlying input were all Greek most likely, with perhaps some local substratal influence.

Alexander specifically almost certainly spoke whatever variant of Greek we accept in Macedonia, but also Attic Greek (the specific dialect of the Athenians in that time period). In fact, the Macedonian aristocracy seemed to rely on Attic Greek heavily, and it would be Attic which would form the basis of the pan-Mediterranean "koine" Greek in the subsequent Hellenistic period. By means of comparison, the differences between Athenian Greek and Macedonian "Greek" were not so really that much more than the difference between, say, Attic Greek and the Doric dialect spoken by the Spartans. The Greek dialects all differed slightly with one another as far as pronunciation, local vocabulary, and some grammar, but were generally mutually intelligible. The Athenian comic poets frequently mocked non-Attic dialects (like Aristophanes mocking Spartan accent in Lysistrata, or the lost work Macedonians by the playwright Strattis mocking the language/dialect of Macedonia).

An excellent exploration of this question is made by Johannes Engels in his essay "Macedonians and Greeks" in A Companion to Ancient Macedonia (Edited by Roisman and Worthington. Wiley-Blackwell, 2010. pp. 81-98). There are a lot of questions to examine to determine whether it is appropriate to consider the ancient Macedonians to have been ethnically Greek, and one of those is their language.

As Engels explains, there are some difficulties in identifying precisely what the Macedonian language was like. There are speeches made by Athenians such as Demosthenes (Olynthiac speech, Phillipic speech, On the Crown) who call Alexander's father and his countrymen 'barbarians' (which was a term they used to refer to people who did not speak their language), but that could have been propaganda. Other ancient writers such as Livy and Polybius consider the Macedonians to have been Greeks. Similarly, studies of ancient coinage and inscriptions have been unsuccessful in identifying how close the language was to the dialects of Greek spoken by their neighbors.

It is most likely that they were speaking some dialect of Greek until it was at some point replaced by the Koine (common) dialect of Greek which became a lingua franca of the eastern Mediterranean in the aftermath of Alexander's conquests. However, until then, Engels concludes that "because of the scarcity and fragmentary condition and/or the bias of the preserved literary sources, the restricted source value of inscriptions and coins, and finally the inconclusive character of the rich material evidence, the Macedonians at least from the late eighth to the early fifth centuries still belong to the many ‘silent peoples’ of antiquity."