Wow! I'm astounded by all these responses. Thank you all for your input, it really is much appreciated, and apologies for disappearing all day - had family to visit and work this evening.
Just to clarify/respond to a couple of points as best I can. (Not in any particular order, I might add, it's late and you all have so much to say!)
Firstly, I am almost certain that all my time and date settings are correct, and I checked against my calendar to make sure it was the day I was thinking of (re going into work during the day).
Secondly, "pestering to babysit" is a bit strong - he offers from time to time to watch DS for me for various reasons - I have very little in the way of a support network round here and also not great at asking for help (much easier to accept an existing offer than ask, iyswim) and I like to think he offers out of friendship and the desire to help, as opposed to getting an opportunity to fiddle with my son.
Thirdly, again I am almost certain he navigated directly to the page based on my viewing the history, which was admittedly at the weekend and I haven't looked since, although that said it could just have easily been a msn link as an email one.
Fourthly, both my partner and I are very aware that sexual abuse is normally perpertrated by someone close to home, sadly, ime, due to personal experience. So please, don't think I'm closing my eyes to the potential dangers. I can't speak for DP on that, I've barely seen him today.
Fifthly (this is getting daft, should have used bullet points) he does play with my son, I've seen this with my own eyes - they play wonderfully together and DS simply ADORES this man, he's always talking about him and desperate for him to visit. It's entirely possible he spent five minutes looking at porn and then the other hour and twenty five minutes (or however long it was) playing nicely with DS. There were only three pages of a porn site viewed, find it hard to believe that's an hour and a half worth of viewing on a free site tbh.
Also think it's not necessarily that odd to find yourself aroused whilst in the company of a child, presuming you're not aroused by the child, iyswim? Could have been bloody Sarah-Jane off Higgledy House that did for him, tbf. She does for DP
Sixthly (not a word I don't think) our flat is tiny, even if DS is in his room pottering about while I piss about on mumsnet I can see and hear him easily, so not a supervision issue per se. I've no objections to my babysitter checking facebook, or playing wow, or w/e, while babysitting, due to size of flat.
To those who point out it is the little things - I know - and I can assure you I will not forget this incident. However, I do think reporting him to the police is an over-reaction at this point.
All that said, I am going to avoid having him babysit at least for a while, as while I am reasonably confident this was merely(!) a catastrophic error in judgement as opposed to paedophilia, I do not want DS exposed to porn and am not comfortable bringing it up with my friend as yet.
Thanks again for your input. Those of you who see something more sinister here than I do, please be assured that I am aware of the potential risks and am acting accordingly.