Hi and welcome! I am sorry for what brings you here but glad you found us. You are in a great place to learn and talk with others who get it.
I just want to help him, I don't want to cause it to escalate
What you described about communication sounds familiar. You are not alone. The best thing you can do to help him is to help yourself. I know, sometimes that is annoying to hear, but (!) it is true. A lot of times how we respond and talk does escalate a dysregulation rather than calm it or even head it off. I don't mean you can control him but there is help.
he will point out every negative and never see anything positive even when I point the positives out
For people with BPD feelings = facts. So if he feels like everything is negative he will talk and act that way. Pointing out the positives seems like the best thing to do but often it is in fact invalidating to the pwBPD (person with BPD) and you will see an escalation. It is a common mistake. Validation can be tricky but you can learn it. In this case, I would say something like "wow, it must feel awful to see so many negatives. That is really difficult" Validating his feelings makes him feel heard and understood and sometimes that is enough to help pwBPD. We have an article
here that you may want to read.
Can you think of anything that may have caused the last 6 months to be more difficult than usual?
Feel free to jump in and start posting. Having a support network will be important for you as you work things out.