Could you advise me on this problem folks?

I currently have a student from Hong Kong staying at mine for 15 weeks as part of Durham University's Homestay Scheme for which i get paid £100 a week. So far so good.
I couldn't get my broadmand onto his laptop, so i just said he could use my bedroom computer at anytime if he required the internet.

However, I soon noticed he was deleting the history every time he used it and it soon became apparent after checking temporary internet files that he was downloading loads of Japanese porn from my computer onto a disk he must have hidden soemwhere.

Now i'm not prudish in the slightest, but do any of you think he's taking the piss here, big style?

I have 2 worries:

1 He's wanking all over my bedroom, firing at the walls,the ceiling, floor, bedsheets, etc and soon it will begin to smell and i will have to redecorate.

2 Porn sites are terrible for containing viruses and i don't wanna lose all my stuff and have to load windows back on.

Apart from this, he's a dead polite bloke, who doesn't invade my space too much. Should i say something to him? knowing he'll be so mortified, he'll probably run to the hills or should i keep quiet and just regularly give the bedroom a washdown and regularly back up my data onto disk.