Jas - I don't ski often at Tamworth, but I do (or rather did - haven't done much lately) teach at a dry slope not far from there. Lessons (and the slopes as a whole) are always busy from when the clocks change until a couple of weeks before Easter. So if you can time things so your son gets his early lessons in the period May to September, and especially as mckenzie says if you can go midweek/mid-day you'll get better value for money (though as most instructors have "day" jobs too there aren't many lessons on offer then). I've often taught "group" lessons with only one or two people in the summer, whereas it's 10 to 12 in the winter - but the price is the same.
As your DS is only 3 and a half, then waiting another few months won't hurt if you're not thinking of going until 2004 (I presume that's what you mean by "next year" - it's quite early otherwise. Some will get it at that age, if they're physically strong and well co-ordinated, with others it might not be until they're 5 or 6. That's not to say that they might not be able to enjoy it earlier - my DS started at 8 months!
By the way - there is another indoor slope at Milton Keynes if you're definately wanting an indoor one. But the outdoor artificial slopes are usually cheaper places to learn if that's a factor.
There are often discounts available on the list prices at Tamworth - I won't post details of how to get them now (since I nominaly work for a competitor!), but ask if you're interested.