Hi and welcome to DTA. When is he chewing on the carpet? Is it when you're home? Or when you're gone? If he's chewing on it when you're home, as busy as your household is, you're going to have to keep a really careful watch on him and redirect him each and every time - not just sometimes, once in a while, when you're not busy, etc. Take him away from the carpet, give him
something he likes better, get his mind off of it. If it's happening when you're gone, then the crate is the answer.
If there are only two particular spots he chews, is there way you can totally block those spots off from him (at least for a while)? I'm wondering what the attraction is, and what he'd do if he can no longer get to those spots? Maybe put a chair, small piece of furniture, small cabinet (cuz he is a Mastiff, just thinking -- he'd probably move a chair

) over top. Out of sight, out of mind? Not sure, just tossing out ideas. Sometimes dogs (or us humans, for that matter) may start doing something maybe out of boredom or maybe for fun, and then over time it becomes a habit, but habits can be broken - sometimes once it's out of sight (over time) it's out of mind. Might be worth a try (if he's only chewing those particular spots, and not chewing carpet elsewhere also).
You say he's going on walks - how often, and for how long? At 7 mos old, he's a large puppy with puppy energy - and he may be taking it out on the carpet (cuz, hey, it's fun). With a bit more info, maybe more ideas will follow.