Have to agree with the previous responses.
The major problem with sex education is not that young people have sex, it's that their parents act as though 'their' child doesn't do 'that sort of thing', and acts as if it's not happening.
The fact that your stepson is discussing the purchase of condoms with you tells you two things - he is already, or is seriously consdering a sexual relationship, and he feels close enough to you to be able to discuss it with you.
He obviously wants to take a responsible approach, so you need to ensure that he is aware of all the forms of contraception, and the consequences of failing to use contraception.
I told my three daughters the same statistic - becauxe the act of intercourse is designed by nature to create a pregnancy, the odds are seriously in favour of that result being achieved. To put that into perspective - if every sperm from the average ejactlation was to fertalise an egg, then the popuolation of North America (the continent that is, including Canada, second biggst country in the world) would double. Then I would pase, and say that it only takes one of those sperm to fertlise one egg - so think about those odds. Intercourse makes babies - unless you intervene in advance.
Have the conversation, and take it from there.
Good luck.