Good idea, John, much of the Trail is low-level and
relatively sheltered, although the Lairig Gate is at 650 m and the approach to Spittal could be exposed or difficult in bad weather. However, with very restricted daylight hours, you'll have no real options about whether or when to brave the rain, and without knowing how robust your son's morale is, (or yours:), it's impossible to say what kind of trip you'll have

Drying clothes can be really hard when you're camping and the long hours of darkness could be challenging. Might be good to keep open the option of B&B for at least a night or two ...
You don't say where you'd be starting from but I'd strongly recommend getting a good 5-day forecast at the last possible minute and then deciding how far to commit yourselves. I researched most of the book in January and February and the photographs prove that good weather is possible, but I was starting from Dunblane so had accurate predictions before setting out. Without tempting Providence, I'd say that the trail is
typically 100% open and passable to the well-prepared hiker in all months of the year, but obviously extreme weather can affect the Scottish Highlands at any time, so check before setting off.

As for the book, Rucksack Readers is open almost 24/7/365 and could organise a copy to be waiting for you anywhere if you phone us on 01786 824696. I did just try to phone Blairgowrie Tourist Information Centre to check that they are in stock (the walk begins/ends in Blair). However, their winter opening times are Tuesday - Saturday 10am - 3pm so 31.12 was no good. In case you want to try, they have (or used to have) a direct line on 01250 872 960 and the email is
blairgowrietic@perthshire.co.uk (all this and more is in the book, naturally!). Dealing with us direct is usually fastest in practice, especially at this time of year when our main distributor (Booksource) is closed (briefly, for stock-taking), so Waterstones can't get hold of stock if they've run out. However, Amazon.co.uk are in stock so maybe this is an issue of New Year delivery dates.
Good luck, whatever you decide to do, and be sure to get our FREE upgrade card for the offroad section Dalnaglar to Kirkton of Glenisla! Do post a short report once you get back please.
Jacquetta