Idk why people are unironically saying "I bet it was just a misunderstanding." No wonder some people think they have never met a gay person in their life. If you miss glaringly obvious facts in fiction, you'd probably miss it irl too.
To put this to rest, this manga is written by a gay man who makes manga about gay youths for questioning youths trying to figure out who they are as their bodies are changing and dating comes into the forefront of their minds (or doesn't, there's good manga about that too). He does good work
So yes, her son IS gay.