Don't let him try it on. Your deposit should be held in a government approved scheme, NOT by the landlord or agents.
Ask him where the deposit is held, you should have received this information when you handed it over. You would BOTH have to agree to any deduction, otherwise it would go to arbitration.
I don't think there are any laws regarding vents, probably only recommendations. Condensation is a problem in most homes, even those that are well maintained. Don't be intimidated by him, you also have rights. If he becomes awkward, seek further advice from the CAB.