Once DVLA have RECIEVED the application then under Rule 88 (or section 88 depending how you read it) he is valid to drive (unless disqualified or medically barred).
DVLA are helpful over this, but sometimes they do get it wrong, so make sure you get a note of who has said what.
You can hand in your D1 (application) at Norey House (or any DVLA office) and they will give you a receipt for it, this is enough to prove that DVLA have RECEIVED an application.
Some police officers have said that you need to have a Rule 88 letter from the DVLA for it to be valid, I've even had one try and say that a DVLA office isn't actually the DVLA so they haven't received it. Both of these are complete and total rubbish, confirmed by both solicitors and the head of the Birkenhead Police Tango Team (who signed the back of my DVLA receipt to confirm this when Police and DVLA computers had a disagreement over my licence).