When bf is away go to neighbor and let him know in no uncertain terms that you're bringing him some unfucked pussy that only his cock can reach.
You don't know how to approach a horny man who often has his dick out?
What planet are you from?
You already know he watches and finds you attractive, given he likes to see you having sex. So you can easily put into a conversation that starts with pleasantries that you gather he likes having a view in your direction.
Just one thing - be prepared that he might be finding your boyfriend attractive - does happen - just have a ring-fence for your emotions if that less-likely scenario proves to be the case...
Just post a small card with your name and number on it....if he rings you can tell him that he's got a lovely cock