Telling lies?

I am not sure if it's my fault or it's just Jay going through a difficult phase. I remember I told him previously to not accept sweets from strangers or anyone else except from mummy and daddy. (I truly forgot about the elders in the family when I discussed this with him). So, the story goes, my sensitive nose detected mint smell from Jay's mouth one afternoon and I asked him gently if he took any sweets this morning. The boy answered 'no' so confidently. When I asked my Mil, she confirmed that he did take a bit of mint sweets this morning. I was determined to make Jay tell the truth, with gentle coaxing, bribes and scolding. He just refused to tell the truth! In the end, when he couldn't take it anymore, he cried himself to sleep. Oh, his new technique nowadays was to change the topic.... "I am hungry", "I want to watch Dora", etc. I refused to give in, so in the end I let him sleep. Thank God, the boy finally admitted when I asked him again after he woke up.(not without more coaxing and promises).
I told him that I would not be angry if he would tell me the truth. I wanted him to trust me not to get angry even if he did take sweets from other people. In this case, I told him, accepting sweets from grandmother is not wrong, but he have to learn not to take too much.
The same incident repeats itself on the second day. He still refused to tell me the truth and insisted that he did not take any sweets. This time daddy got angry and decided to teach him a lesson. We would bring the sister out and leave him with grandmother until he realize his mistake. After crying so pitifully and realizing that he's not going to win, he admitted to my questions by nodding his head.
But victory is not on our side yet. I decided not to ask him questions about sweets for the time being (though I know his grandmother will give sweets to him almost everyday). After a few days, I tried asking him what he had for breakfast.... And he refused to answer my questions! He kept trying to change the topic. I don't understand! I just wanted him to share with me what he eats in the morning, and what he did while at grandma's house. He would sometimes tell me about the things he did, but he refused to tell me what he eats. Can someone tell me is this normal for an almost three years old?
I have been reevaluating myself, and I thought the only reason could be that I am more particular with what my kids can or cannot eat. Since Joy in on the Gfcf diet, I am quite skeptical about giving too much of the food to my boy too. Hmmmm..... Anyone with ideas how to handle this? With my eldest affected with autism, I felt as if I am still new in parenting with my second child since my daughter did not go through the same phase. :(