Hi, I'm a single mum too of a teenager, tho 19 now, off to university soon, working and travelling.
I'm with LoneFather on this, best to be open about everything.
I agree all (most) teenagers see porn sooner or later, I did at the time (magazines).
I would have a conversation (ongoing) with him about appropriate responses, behaviour, growing up, becoming an adult, porn in the media and internet and how it influences society (the pornification, as they call it)have you had the condom talk? Very important!
Girls, etc.
He must have women/girls (granny, aunts, friends) in his life who give him the right way for his attitude towards women.
Also explain to him if you are going to do it, why you are accessing his internet usage to curtail inappropriate content.
If you are open to your child, and by talking to him, encourage that in him, you can talk about anything! (We do)!
All the best! Single mothering a teen is a whole lot harder than a toddler/child!