It could be either picky eating or he could not be hungry. I have found the eating habits of toddlers to be very unpredicatable and always changing.
Our pediatrician and those of friends and family all say the same thing - he will eat when he is hungry. They also suggest not making a big issue about it and when he does eat not forcing him to clean his plate but rather stop when he is full.
My oldest just turned 3 and there are plenty of days when he skips a meal. My 2 year old is the same. I make them sit at the table with us and offer them food. I try to make sure there is something on the table they like to try to entice them to eat. I don't make them eat everything, however, if they eat only a couple bites they aren't allowed dessert. I try not to give them amything to drink for 30 minutes before a meal. I've noticed they can fill up fast on liquids making them not want to eat.
My sil puts her daughters' plates in the fridge and offers it to them when they tell her later they are hungry. Her rule is you can't eat anything until you eat your dinner.
My kids are also big on crackers and pretzels and chips. Sometimes, when they ask for them, I tell them it is time for fruit or yogurt instead. Sometimes I go as far as to tell them we are out of the item they want. Good luck!