It seems you are quite open with your nephew so id keep things honest but light hearted. If he turns up in tight shorts tell him "for god sake put it away" and same if he offers to let you have a look, tell him "Im your aunt, that could almost be considered incest".

But yeah, id make it somewhat of a joke so he dosent feel embarressed and awkwasrd about coming to you in the future. For now i would also leave your boyfriend out of the situation as it might make things even more uncomfortable, but if tthe nephew gets worse do get your boyfriend involved to try and sort things out.

You done the right thing giving him a condom to try and tellhim if he needs it you will even buy him more but he should work it out on his own