My pup'is 9 weeks old and has very short hair and very little hair under arms. Will it grow longer or should I get him a sweater for winter months to keep warm?

Both dogs have shorter type hair. His hair probably won't get any longer than what it is now. A sweater would be a good thing, especially if your winters are cold. Just remember to take it off as soon as you get inside or if he looks like he is panting a lot.