Sorry but I really think your priorities are all wrong. Sex really is the glue that keeps a relationship going when the going gets tough.
You think you know the extent of the debt but I'll bet you don't have the first idea about it. He's just put down what he thinks will be discovered.
You are committing your money and YOUR MOTHERS to someone with a proven bad track record with debt and a liar to boot.
Have you been honest with your mother about the situation and if so is she still happy to throw give her money away?
Are you really ready to take on a life's worth of debt? This situation will only get worse and is potentially something that you will never, ever escape. You are going to blight your future with this man.
My ex was a gambler, endless loans etc. The only saving grace of my marriage was that the house was mine, and he had not contributed 1 cent to it, so it stayed mine, and that we had no joint debts, because I refused to sign up for them because he was an arse. We had a joint account, to which he had no access, apart from a paying in book.
You need to shut all your joint accounts down and reorganise your finances as of yesterday to protect yourself. You also need to have the money you put into the house ring-fenced(if you really are going ahead with this - which is honestly mind blowing.)
You are basically giving him access to a huge asset, which he can then run up yet more debt against.