I'm not sure of the exact year the Egyptians started worshipping Amun but he was an Egyptian God at the time of Alexander's conquests.
I don't think I stated Alexander was the only one to link the two Gods together (if it seemed that way I apologize) I think it was a pesonal decission and not one that was forced upon anyone. Your post made me do some more searching for information on when Amun was first worshipped and I found some things that seem to match what you had posted:
Ammon was an oracle god, whose oracle was situated in the Siwa oasis, some 500 kilometers west of Memphis, the capital of ancient Egypt. Originally, this was the place where the Libyan desert tribes worshiped a god who had the shape of a ram. He may have been related to Baal Hammon, a god venerated by the Semitic peoples (e.g., the Phoenicians and Carthaginians). However, this is just a hypothesis, and we can not be really certain about the origin of this cult - as is nearly always the case when we discuss an aspect of ancient religion.
The cult was taken over by the Egyptians, who identified the god with their supreme god Amun; they called god of the oracle 'Amun of Siwa, lord of good counsel .
Another center was the Macedonian town Aphythis, where the young Macedonian crown prince Alexander must have seen the statue. When he had become king, he visited Siwa (February 331). According to Arrian of Nicomedia, Alexander did this because he wanted to imitate his legendary ancestors Perseus and Heracles. This is an odd couple: Perseus never played a role in Alexander's propaganda. However, since the fifth century, Perseus was regarded as the ancestor of the Achaemenids, the Persian royal house; and everybody knew that the Macedonian kings descended from Heracles. Following in the footsteps of Heracles and Perseus was therefore, in a sense, a religious preliminary to the conquest of the Achaemenid empire.
It is possible that Alexander had already started to venerate Ammon, because during the sack of the Greek town of Thebes, he ordered that the house of Pindar had to be spared. On the other hand, there is no evidence that Alexander worshipped the ram-god before he visited Siwa.
However this may be, the result was important: Alexander was greeted as Ammon's son, and started to believe that he was a demi-god indeed. According to an admittedly hostile source, Ephippus of Olynthus, Alexander sometimes wore the horns of his divine father Ammon on public occasions. We can not establish the truth of this story, but it is certain that immediately after his death, he was depicted in this fashion.
www.livius.org/am-ao/ammon/ammon.htm
Until the Middle Kingdom his influence was local; but when the Theban kings had established their sovereignty over Egypt, Amen became nationally pre-eminent as Amen-Re, and by the 18th Dynasty was called the King of the Gods. His famous temple, Karnak, is the largest religious structure ever built by man.
Amen according to the older Theban traditions, was created by Thoth as one of the eight primordial deities of creation (Amen, Amaunet, Hah, Hauhet, Nun, Naunet, Kau, Kauket). Later traditions cast him in the role of self-created creator, who shaped the ordered world out of chaos through masturbation and self-fertilisation.
As creation god he assumed at times the name of Kematef (Greek Kneph) and was depicted as a snake.
As the Egyptian state god during the expansionist period of Egypt's history, Amen was the god to be thanked for the military successes. This was done both by endowing his temples with vast wealth as well as through the offering of severed hands and penes of fallen enemies [2]. Both penis and hand were symbols for Amen's powers. Amen's priestesses, the Wives of the God, were also called the Hands of the God. The obvious thought association arising from this epithet is not supported by any evidence. Very little is known about the Amen worship but what happened in public, such as the yearly transportation of Amen's statue from Karnak to Luxor, or the annual peregrination of the pharaoh and his wife to the Amen temple and their penetration into the inner sanctum.
http://nefertiti.iwebland.com/religion/amen1.html
I long for wealth, but to win it by wrongful means I have no desire. Justice, though slow, is sure.
"Solon Fragment 13" poem