Hello, I am also a boy, I recently reach to C # 6.
The general feeling is that the entire passage is smooth. First shut the mouth and“ hum hum ” in nose with a false sound, and then slowly open your mouth, I feel the passage is smooth. The higher the pitch, the more effort with the upper jaw, I don't know if this can help to you. However, I can only pronounce "o" or "ah" in this case, which is hard to say words.
Before this I was able to ‘suck out’ the G6-D7 dolphin sound, not very stable, only ‘sucked out’. I just want to complete my range of sound.