I need [Help] with my boyfriend

My boyfriend is planning on going to see someone soon but in the meantime I really need some help. He is 19 and has an extreme social anxiety issue. Most of the time he is fine with one-on-one or small group interactions but when he is in a large crowded public place he has panic attacks. Today he called me while he was at a concert because he had to leave the area because of the noise and people. He always looks to me to help him when he gets like this but I honestly don't know what to say. If anyone who has this problem knows what I could possibly say to him to help him calm down that would be great. Most of the time I tell him he will be fine but he seems like he's getting frustrated at that answer and it's harder to know what to do when he calls me. If I'm there I can hold him and it usually calms him down. I guess I'm looking for something that's more specific to tell him when he needs to calm down.