Hey Femme
First of all you not the only girl here, i was with my bf for 2 years, when we first started dating he was typical 16 year old booy who'd get an erection at the drop of a hat. He did the same thing tried to hide it from me. I think what you said you were gonna do, just let him know with out saying anything is the best bet, after a while when my bf was still pulling away i just quietly said its ok so only he could hear, and after that he was fine then he didnt get them as much i spose because he was worrying about getting them before i said it was ok. Anyway thats my pearls of wisdom, and have fun with it.
Luv Ya
Tilly