JerichoJax and Welcome.
Is this something others see with their ex-partner? Any clues why?
Yes, this is very common. I have many theories why and non of them good. I think it may be helpful for you to think whether friendship is something that would work for you?
He has sent me pictures of their older dog tags he apparently took and has them out almost like decorations. What is this all about as it doesn't make any sense to me?
He also took a decorative item we had wine corks from bottles of wine we have drank. Why would he want these things?
I have read quite a few stories talking about this and I think it's a lot to do with object constancy, the items will remind him of the dogs and give him a sense of closeness. He may have taken the corks because they remind him of good memories.
He is routinely on a hookup app even having a profile line offering to give guys oral sex. While together he told me he wasn't really a sexual person, that sex was just physical and in the last two years we hardly had sex. Giving oral sex is also something he wasn't into. Sometimes I feel he is doing this to get me to contact him while other times I feel like this is some form of self harm or something. Any clues?
A pwBPD has an unstable sense of self and this can cause sexual identity issues, he may be struggling with this. It could be seen as self harming I guess because it can be seen as self destructive behaviour.
You probably have a lot of questions and I look forward to you posting more.
LT.