Depending on where you are located, the foster owner may be right. When we first got Nellie, who is thunderphobic, we could not even get her to go near the door to go outside when it was raining, or if the storms were 12 to 24 hours away. We would have to leash her just to get her outside. If there are still storms coming into your area, he may be reacting to the pressure change associated with the front.
He may need some more time to get used to you and his new home. It took Nellie a few months to decide that she didn't have to hide when she felt a storm coming and would at least stay downstairs with us. We have had her about 4 years now, and she will still go hide during the storms, but will stay downstairs until it storms. She will still not go on a walk if she believes a storm is coming and will just sit in front of you and keep moving to stay in front of you to get you to go back home.
Our last border collie was fine with just about anything, until she was home alone during a wind storm that blew a tree down in the yard. After that when the wind would pick up, she would get very nervous and go to bed.