Too toxic to forgive?

With same guy for several years and he has a history of always underestimating his financial matters (tells me he owes less than he does on cards, etc.). I have a nice nest egg I've built by working my whole life and saving. I'm a few years from retirement and he wants to marry me (or at least live with me in a few months).
While he is much better at budgeting (because I've taught him and he knows I do check from time to time) and he appears to be more truthful in some of his finances (what he owes on his cards but maybe b/c he is using some inherited money to finally pay them off), he stills lies about certain things. For example, he is the POA for his elderly mother and he used her cards (he says with her permission) on numerous occasions when he was unemployed for various items some of which are food (which I understand) but also liquor and car washes (that's not critical in my mind). He understated how much he used her cards when I asked and also understated what she still owes (which she can't afford to pay in full). He mishandled her cards in terms of allowing late fees and interest to accrue.

Now that I continue to confront him, he has sent one of her accounts to a lawyer who will negotiate it down and the other one he pays a decent amount b/c he wants her to have at least one card should she need it (but hopefully he won't use it for his own now that's he employed again and collects social security as well).

But last week I asked him about what he is doing to address his mom's credit cards and inquired as to the balance of her 1 card he is paying with her money. While I know the amount is about $7,100 (with added late fees when he didn't pay it on time a few months ago) and ongoing interest, he said "about $6,000" to me and seemed reluctant to say anything to me. WE HAVE AN EXPRESS UNDERSTANDING THAT TO MOVE FORWARD WITH THIS RELATIONSHIP EVERYTHING MUST BE TRANSPARENT.

Since I confronted him, he has been paying the cards (partial but decent payments) and isn't using his mother's cards. I suspect he is doing this now to appease me. How can I trust his behavior has changed for good?

Therefore, the question is: do I just let this go and move forward with him or is this just another example of his persistent lying and finally give up even though we love each other????