Without seeing exactly where he has parked the van it's impossible to tell if he has parked legally or not but generally speaking if there is free parking on the street and he has parked the van facing the direction of traffic, then unless he is within 10m of a junction, blocking a driveway or access road or impeding traffic flow then in the eyes of the law he has no less right to park there than anyone else.

Parking rules for vans (but not HGVs) are no different to cars - any vehicle parked in a zone where the speed limit is over 30mph should display a parking light (car or van) but the Highway Code distinction between 'should' and 'must' is advisory versus mandatory. Legally, he doesn't have to.

It would appear here that no laws are being broken. If there are genuine concerns about the van inconveniencing local residents the driver may not be aware of this, in which case you could leave a (polite) note under the wiper. However, there is nothing you can do to force him to park elsewhere.