For Westvleteren I strongly recommend a satnav or smart phone with google maps. The monastery is down a number of small roads and although it is signposted you'll find GPS very useful.
If your husband is a Belgian beer aficionado he probably knows this, but the only way to get beer direct from Westvleteren monastery is by booking in advance by phone and buying a crate (or two) of 24. You can only get the type they have that day. If he is after a crate then good luck, I've never succeeded in booking despite many attempts.
Assuming you don't want a crate what you need to do is go to the monastery visitor centre 'In de Vrede' where you can buy all three beers for consumption on site. The gift shop sells boxes of six bottles of whatever they have in stock, maximum two packs per person, and they might not have any at all - it's a matter of luck, That said I've been three times and never left empty handed. I have managed to buy all three but the 12 (which is the one most people want) is all they had on my last visit a couple of months ago. Nice light lunches too. Be careful as In de Vrede isn't open every day especially in the autumn - check their website,
Again he may well know this, but St Bernadus 12 is made to the same recipe as Westvletern and easily found across Belgium at a much lower price.
If you're driving to Westvleteren I would also recommend a visit to Ypres nearby and to one or more of the WW1 war graves in the area - absolutely worthwhile. Back on the subject of beer if he wants to see a decent range then I recommend Beer Vanuxeem in Ploegsteert near the French border - many hundreds of beers at low prices available singly or by the case.
All easy quiet back roads, a nice drive.