Okay guys, so here's the update. I stopped messaging him altogether and then this week he wished me a happy birthday and told me he'd likely be posted back for good in the new year and would like to take me out for a drink if that's the case.
The woman who posted pictures of them together on social media is no longer linked to his social media so I presume they've fallen out.
And he's been messaging me practically daily ever since. I questioned if he'd been seeing anyone and he said no. I so told him I'm not interested in anything casual and he said it wouldn't be casual as he'd be open to moreonce back, not now.