Lyle's iPhone 5s keeps switching between SMS and iMessages. Leo says that iMessage is best if Lyle and his friends all have iPhones. Leo says that WhatsApp is an option, but he'd have to get everyone to use it. Cross platforms can be tricky.
SMS is the standard, and he can choose that setting for those who aren't on the iPhone and iMessage for those who are in his contacts. He should also check to be sure the option to send as SMS is turned on, should iMessage fail.