*TW* Advice for assaulted boyfriend

It’s not me that was assaulted, but my boyfriend has (I know of two times). The first I know was on a school field trip to Spain and involved a male roommate, and the second was with an ex while he was drinking and was asleep (I don’t want to go into more detail for obvious reasons). We’ve been dating for over a year now, and with things going on we decided to get more intimate (we’re both over 18). It wasn’t the first time we have, but this time we felt it we were ready to actually have sex. It was going really great until he had a ptsd episode and had to stop right before we actually started. I was making sure the entire time beforehand that he was comfortable and ok with what was going on, and he was fine up until that moment. Thankfully he told me to stop, and I calmed him down. I don’t know exactly what he’s going through, but I want to be as understanding as I can be because I want the best for him. Any advice to help in these moments?