Growing pains

Do anyone have any experience with growing pains?

My 9 week old newfie/aussie pup has grown quite a lot in a week. 2.2lbs in a week, I've had to loosen his harness twice and the vet says he is a bit thin.

It's not to bad yet, generally just a bit of a hard time settling down, some limping after waking up and generally just annoyed with his hindleg.

He doesn't do much, some quiet play, training, socialising (mostly just sitting and looking at stuff and people) and few short walks where he usually ends up being carried. And he insists on sleeping like Superman.

I mostly just wanna get some advice, in case it gets worse or if there is anything that could ease his comfort?