Nuts, vegetables (have minimal carbs so they count as free), and popcorn (in moderate amounts) are also free. Crystal Lite is a good alternative to juice or lemonade.
Free foods are any foods that don't have (or have very small amounts) of carbs. Any foods that don't contain starches or sugars are free. Dairy products contain lactose, which is sugar, but cheese is eaten in such small quantities that it is considered free. Fruits, grains, and anything with added sugar has carbs. Do your inlaws understand how diabetes works? If they don't, I would start by explaining the basics to them, and then explain why your son has to be careful with eating carbs. While he needs carbs, he needs to balance them with insulin, and so free foods are like safe foods for him. Otherwise, hopefully the starches/sugars thing makes sense to them! Just remember when explaining to people that it can take a while for others to understand, and may require lots of repetition of information. Try not to get too frustrated and don't blame yourself for anything, no matter what they say. Best of luck!!