Good question. If you read it carefully, you will see that Jesus is telling him that he is wrong. This is just my personal observation and not the official teaching of Jehovah's witnesses.
However, even if he were calling Jesus his god, that would be perfectly proper. If you remember, God said that He made Moses "a god"--"as God" to Pharoah, and Aaron would be Moses' prophet, making him God to Aaron, too:
"And the LORD said unto Moses, See, I have made thee a god to Pharaoh: and Aaron thy brother shall be thy prophet. 2Thou shalt speak all that I command thee: and Aaron thy brother shall speak unto Pharaoh, that he send the children of Israel out of his land." (Ex. 7:1-2 King James or "Authorized" version)
Young's Literal Translation shows what the original Bible manuscripts actually said:
"And Jehovah saith unto Moses, `See, I have given thee a god to Pharaoh, and Aaron thy brother is thy prophet; 2thou -- thou dost speak all that I command thee, and Aaron thy brother doth speak unto Pharaoh, and he hath sent the sons of Israel out of his land."
The Amplified Bible explains:
"THE LORD said to Moses, Behold, I make you as God to Pharaoh [to declare My will and purpose to him]; and Aaron your brother shall be your prophet."
So God, Himself, plainly said that Moses was to be a god, as God, to both Pharoah and Aaron, his brother. Of course, this meant that Moses would be God's representative to Pharoah and to Aaron.
Now, do you remember that Moses pictured Jesus? Both narrowly escaped death as babies, both saved God's people, both were mediators for God's people, and both were mediators of covenants for God's people:
God told Moses, "I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him. 19And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him." (Deut. 18:18-19 KJ/AV)
The apostle Peter showed that the 'prophet like Moses' was Jesus, at Acts 3:20-23 (KJ/AV):
"And he shall send Jesus Christ, which before was preached unto you: 21Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began. 22For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you. 23And it shall come to pass, that every soul, which will not hear that prophet, shall be destroyed from among the people."
So just as God Himself made Moses "a god"--"as God"--to both Pharoah and his brother, so Jesus is rightly, as the 'prophet like Moses', "a god--"as God"--as God's appointed representative, to both his enemies and his brothers, including Doubting Thomas:
"he is not ashamed to call them brethren, 12Saying, I will declare thy name unto my brethren, in the midst of the church will I sing praise unto thee." (Heb. 2:11-12 KJ/AV)
Also, if you check Strong's Exhaustive Concordance, under "God", using the reference numbers to refer you to "elohim" in the Hebrew dictionary in the back, you will find that even magistrates were called "elohim" or "god(s)" in the Bible, that this was normal and simply a gesture of respect, as we say, "Sir", from "Sire" (Father), out of respect.
Now I have a question for you. Since the Jewish Bible has God's name, "YHWH"--Jehovah or Yahweh in English, all 6,700 times where it occurs in the real, original Bible, why do most people insist on using a Bible that has taken God's name out of His own book and replaced it with merely a title, "the Lord," or "God," all or almost all of those 6,700 times?
Best regards,
Mike