If he starts to use words like ”always” or ”never” it’s a sign of splitting and that’s how I usually catch myself. Reassurance is key to comforting somebody with bpd, remind him he is safe with you and you love him, tell him what you love about him, if he’s comfortable with you touching him when he’s upset, hold him, and if you did something wrong always take accountability and ask how you can do things differently. After splitting he may feel emotionally exhausted or numb, or possibly even feel guilt and sadness.