My DS was tested at 3 1/2. Not for giftedness but other issues. I didn't know he would be given an IQ test so didn't even talk to him ahead of time. The tester did let me come in the room, though, so I could see that DS was not trying. Luckily I went in because I do not think it was apparent at all to her that DS was giving her wrong answers on purpose, whereas it was completely obvious to me given the smirk on his face. But I couldn't really say or do anything about it. After a while, he simply refused to answer any more questions and was under the table meowing like a cat. In the middle of the test, at one point, he also took her keys and was going around the room trying all the locks, like her desk drawers. I think he spent quite a bit of time lying under the table rather than sitting in the chair, and she was asking him questions as he was lying down.
When she got to the point where he wouldn't answer any more questions she told me I could bring him back another day. I decided that unless she saw something glaringly wrong with his cognitive ability, that would be pointless. I wouldn't trust the test results anyway. I was shocked when I got the test results in the mail along with a report and the report didn't say anything about the behavior or that she stopped the test! She scored it anyway! His full scale IQ was fine (like 106--now it's a lot higher), and since we were looking for developmental problems maybe she thought it was fine to score it. I don't know. The school system wanted that report and I couldn't give it to them because I knew it was inaccurate information.

I don't really have any advice but it sounds like he calmed down enough that the second session should go a lot better.