One thing to consider is what does the contract you have with him state? Does it specifically prohibit the use of the van for personal use and if it doesn't does it specifically say what the penalties will be?
The other thing to consider is are you 100% sure that the way he worked for you can't be interpreted as employed by HMRC etc? If there is any chance they would think they were employed then you could be in for massive back tax, national insurance, holiday pay not to mention the fine!
If he's a subby then he can work when he wants, so a sicky isn't really a thing is it? From some of the stuff you said I think HMRC would consider him employed!
While you may well be morally right, how much could it cost you if you had to go to court, let alone if you lost!
Might be best to consult a legal professional to be sure as this could get very expensive!