Not pulling out

So my bf and I are long distance so we sext pretty often. The last few times while sexting he's talked about not using a condom and not pulling out. (Because we were sexting and I didn't want to ruin the mood I always just went along with it) This might be tmi but he's said the thought of it "overflowing" is something he's fantasized about. So next time I see him this is something he wants to try and idk how I feel about it... Now I do have the nexplanon in my arm so I'm not completely unprotected but the thought still worries me. The only reason I haven't told him no yet is because I feel like I let him down sexually so often I hate to do it again...And I don't really know what to do. What do you think the chances of me getting pregnant are if I do let him?