well he WAS bitten - your flatmate saw it happen!
it may not have seemed much at the time, but presumably it was worse than it looked and has now caused an infection.
i assume this is painful and worrying and also costing him in treatment - painkillers, antibiotics bandages etc - so i dont blame him want to recoup some costs.
presumably they are company uniform trousers - they may be a light colour and now stained with blood - and for the dogs tooth to go through his skin, it is likely it made a hole in the trousers - no matter how small - the company wont want him making deliveries in that state
if he was to launch any claim, he would have to prove the bite - photos, hospital bills etc, verification from the vet and a doctor etc - you cannot just make a claim like that without evidence.
his text is meaningless as he is entitled to change his mind
i agree him asking you out is a bit weird and innapropriate - but some blokes are just like that - will try as much as they can, all the time in the hope that one day, one girl will say yes - maybe you seemed so sympathetic and kind that he thought you seemed nice.
maybe he was a bit drunk and emboldened.