That can happen with any breed if they are afraid of storms.

Try turning on some soft music and speaking calmly to him. If he's under the couch don't drag him out, just sit by him and talk calmly to him. Be careful petting him in case he is really terrified and might bite you not meaning to.

If he's that afraid of storms you may want to ask your vet for some tranquilizers. We have alot of clients that do this (various breeds) and give them when they know a big storm is coming. It doesn't put them out but it does calm them down so they don't panic or hurt themselves until after the storm goes through.

The best you can do is just be with him, stay calm, and reassure him. I'm sure others may have even better suggestions of other things you can try.