You need to control the conversation and only select from the friendly options until they have built up friendship. Once friendship is built up to a good level their autonomous chats will tend to be friendly too. Helping with school projects gives a nice boost as does watching stars (skies) without you having to control them the entire time.
This probably may have something to do with their traits, for example, if the mother, and even the teenage sim too, has a trait to be moody, angry or evil, which enables her to do things that the teenage sim may not like, or embarrass him in the game (and real life too) :D. Or it just has something to do with a glitch or a problem in the game system. Try to make them go to some events together, as a family, in the hopes that they might find the link between them.
