@LurkioJr as I see it, the big problem here is your mindsets.
You went in thinking, “this will give us a joint asset, make him/both more committed to a joint future and is a step towards more togetherness in future, possibly joining finances, marriage, children etc”. With that mindset in your mind, you seem to have approached it generously, giving him a £16k share for £16k (although not formally, because you presumably trust him and want to build a life with him) and you then spent a further £26k of money of which he didn’t match, leaving aside the later £9k that he might pay for now. Presumably you spent all that money again as you thought it was an investment in your joint future, the same way you don’t (or shouldn’t!) ask your husband to repay you for the milk you bought for them, but you would be well within your rights to ask a flatmate to do so.
So far, ok.
But the current uncovering of his financial position shows his thinking loud and clear. He didn’t give you the full picture on his finances to let you make a decision about the ‘relationship discount’ you were giving him, he only said enough to let you know in broad brush strokes that he was ‘slimy’ or not. He then saw you spend all the money and didn't offer to contribute, even though you spending £23k as presumably improved and increased his asset as well. And finally, he continued to build savings at your expense without you knowing about it.
Who was going to fund him when he was possibly out of work?!?
Regardless, it doesn’t appear that his mindset was one of an eight year relationship, moving closer and looking at building a joint financial one. It looks like he was business minded and came out of it extremely well. Can I criticise him for that? Not really - you should have been equally as business minded and not so hasty to give away or dilute your largest asset!
Overall though, if the man I loved and had been with for 8 years did this, I wouldn’t trust that he thought we had a future together. I would insist on buying back my £16k equity and sole ownership of the property, and once I had secured that, I would quietly think and assess the whole relationship. Maybe he is fine and it’s just this one bad thing. But on the other hand, maybe you always paid the mortgage and he just did half bills. Maybe you always bought the cushions or the new linen for the bed. Maybe you always buy the groceries, and you just haven’t noticed how YOU are always giving and contributing and he is always holding back and taking.