Aria hasn't had a panic attack before. Aria has significant attachment issues. The thing that sparked it this time was the belief that his mother was going to replace him with her new baby and won't want anything to do with him anymore. I guess it's almost Stockholm Syndrome in a way, in that no matter how abusive Mommy is, at least she's giving him the attention he craves. Now, he fears that attention will be withdrawn altogether and he doesn't know how to process that information, that fear he's going to be abandoned by the most important person in the world to him, next to his brother.
Anyone who wants to be licenced to do close protection has to attend an accredited training school. The training includes first aid training. He's also done a full first aid course and has experience in field medicine. When you're working special ops, you're often way off the radar. There's no one to help you but the other men in your unit. As Drew says, it's not uncommon for soldiers to have panic attacks. He's also received treatment for them himself.
As for the motive...well that will become clear in time.