If your son is 15, he's a real person with real feelings and I think it's time to let him explore other parts of his personality and emotional life than just positive feelings. Noone is positive all the time and pretending like you are probably isn't healthy, so let him express himself. You know teenagers, they need to figure themselves out and it's just part of growing up.
The line itself is negative. I'm not sure I fully understand the lyrics, but is it so bad not wanting to "waste" time on someone who is a negative influence in your life? There are times when you need to stand up for yourself and your interests and maybe such feelings can be a strength rather than a weakness, even if they are negative.
Also, if you've managed to instill positive values into him for 15 years, then they are already deep rooted and I wouldn't worry much about a song.
I did way worse in my teens and I'm a kind soul
