Originally Posted by sparklepop
He wants to text every day, go out together alone, talk on the phone, be involved in moments between GF and I and wants me to verbally reassure him of my platonic love for him.
What comes to mind is someone with Aspergers or mild Autism. Some tend to cling and fixate and don't realize that it comes across kinda creepy. You might want to look up how to deal with people who exhibit similar tendencies to what you witness (I believe you did mention he has mental health issues). It might help. Sounds like he is feeling left out.
Opps, didn't see the last few paragraphs of your post about a personality disorder. Still research and learn how to navigate the situation.