Jaclyn, I can understand the stress you're feeling, and I hope I can calm your worries. Children as young as 2 can become finicky eaters. Some of it is exerting independence. Another reason is they just don't need large amounts of food because the toddler years are times of slower growth as compared to the first year.
When considering your son's situation, remember that he started eating table foods around the same time you had his sister. Much of the energy you put into his needs was now being shared with the new baby. By developing pickiness, he may be trying to pull your attention away from his sibling, especially if he is aware that you worry about his eating. Secondly, young toddlers don't like changes in their menu, so they develop favorites, and that's all they will accept for long periods of time. The foods he ate a few months ago were all saltier foods, but now he is switching to sweeter foods, which indicates a maturing in his taste buds and a change in preferences. After all, even adults will do this, especially with snack foods. How many times have you eaten potato chips and then had a craving for something sweet?
It sounds to me that your son is starting to develop an interest in other foods because he is changing weekly, and this is normal eating behavior in young children. Don't worry so much about portion sizes unless he is losing weight. Children are very good at self-regulating their intake. If he is hungry, he will eat. Toddlers are busy little people and tend to graze on many little meals each day.
As for removing the fruit bars, I approve of this action. Reading labels can show that many of these convenience foods are high in calories with little or no nutritional value. One suggestion would be serving whole-wheat pancakes with applesauce or peaches instead of syrup. Make a whole batch and freeze them. They defrost quickly in a microwave, are nutrient-dense and can be served flat or rolled up like a burrito with a filling.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.