Sweatpants – fabric intended to have stretch?

Now that the weather where I am is threatening to come down out of the high 90s, I'm thinking about winter sewing for my guy. I've made him several pairs of these pants previously, and have always used sweatshirt fleece. They've always seemed very slim, and last year he grew out of his new sweats almost immediately. This year I was just going to size up from the start, but it occurred to me to check the fabric guide - the sweatshirt fleece I've used is knit but has virtually no stretch. There's no stretch requirement for the pants fabric, but can anyone confirm if the pattern is intended for some amount of stretch in the fabric? On a related note, anyone got a source for reasonably cuddly french terry? The only stuff I've found locally is *very* lightweight, which is just not what I want in sweats. ETA: meant to note that my guy doesn't have particularly big legs - he's still comfortably in 2T at just over 3 years old in the Art Museum trousers, for instance.