Hope this is okay, significant other just got diagnosed and I'm trying to understand. I have my own stuff and coming from a place of total nonjudgment.

I have a little childhood baggage myself but he went through horrible trauma and is just starting to deal with how it has affected his adult life. I think it went under the radar for a long time because teenage boys and young men are expected to be moody, impulsive, and most of all hide it. But he and we are just starting to learn about solutions. 

I've been reading on my own but one thing I've noticed about my SO is that he exhibits childlike behavior especially when stressed. I have spent a lot of time working with kids and I finally realized his behavior makes a lot more sense if I imagine it coming from an 8 year old. I noticed he reminds me a lot of MYSELF as an insecure and lonely traumatized child to be honest. I have not seen anything online about this re BPD. Is this a symptom, a coincidence/overthinking, or maybe more generally relates to his childhood trauma? 

For example, he is preoccupied in his head a lot and in that way not as observant as some adults. I feel like our relationship is as a parent and kid where the kid would never be responsible for even noticing the adult's bad day or sad mood.

He likes to kind of climb on me or shadowbox me instead of giving a hug, I always joke he's like a baby monkey

The other day I was upset about something of my own and started crying and asked him for comfort, and he was taken aback and alarmed and could not respond by offering me comfort. I was really confused by this until I applied the "what would an 8 year old do" thinking and realized he reacted like  a kid whose mother was sobbing. I normally have more of the caretaker type role. 

His stubborn logic is very hard to deal with sometimes and it's very hard to reason with him, but that is more generic I guess. He can also be really emotional, giggly, mopey like a child and has a hard time regulating his emotions like a kid, but again that is more generic.

I understand kids and have worked with kids with trauma, and I think a good way to empathize with adults when it's difficult is to see them as once a kid. Is there a frame of reference for this as to bpd? Is arrested development or reversion to childlike responses normal for this disease?