What if I get an erection during my penis examination?

That would be normal. Boys and men can get erections with or without their penises being touched (such as in an examination). That is what happens in normal people. Think of the question, "What if I gag when the doctor examines my throat with a tongue depressor? " Likewise, boys and men can gag during a throat examination... also normal. So, erections are no big deal (you don't have to say anything if it happens).