Why didn't Sonny get on the bus

Why didn’t Sonny get on the bus

I was really shocked by the end of The Flowers. I would have totally expected Sonny to get on that bus, but in the end he did not. I did not like the ending because of this. I think that Sonny probably did not get on that bus because of he was too young he probably was not that in love with Nica. It was nice of him though to help her leave to Mexico and give her money. Yet, at the end when Nica is leaving he tells her that he loves her in French. So this maybe was not a reason why he decided not to leave with Nica. Another possible reason that sonny did not get on that bus was because of his mom. He might have been scared for her to leave her behind. It is clear that Sonny was more mature and responsible than his mother. Or a last option could simply be that Sonny did not want to go to Mexico. Throughout the whole novel he would say that he wanted to go to France.