Did Mr. B not explain what happened when he dropped Jim back home? If dad was thinking Jim was going for a ride and then he came back unexpectedly early, it seems like that would have been the time to explain that he had a panic attack. It opens the door for Jim's dad to talk about any issues his son has that he thinks you should know about. And if Jim is spending a lot of time with you, I would want you to know these things if I were in his position. If you feel uncomfortable bringing it up now, you might just call or stop by to check on Jim and let his dad know that you are concerned about him. Also, his dad, who probably has been distracted with his own grief over this time, may not know exactly what's going on with his son and could benefit from the knowledge that everything isn't as ok as he thinks it is. My son is this same age and I can tell you that you only get little clues to the big things that are going on inside. I would want to know.
The fact that his mom died by suicide is, I think, a red herring. At least at this point anyway. Does Mo know that Jim's mom died? That seems enough of an explanation of the behavior as is necessary.
A really great possible outcome of this friendship is that they each have someone that they can tell their "stories" to without the same kind of backlash that might come from telling kids who have never experienced the kinds of losses that these two have.