Sorry for all the grammer errors! Have to check before i post!
it's possible it could be anxiety? maybe when he feels the need to go he panics slightly, making him feel like he needs to go more than he actually does (i.e in the car). If this is a continuous things he maybe constantly anxious as he know what will happen the next time he needs go (wetting his pants, or being so anxious he can't actually pee). Possibly try going back to square one with him, get him to tell you when he needs to go, tell him he's not going to wet himself etc. also make sure he goes before he goes to bed (which i'm sure you already do). I'm prob way off the track but obviously you've gotten him checked for infections etc. As for your one year old, the only thing you can do is reinforce to your son not to squeeze her so tight and that she only little. hope all goes well! :)