Honestly, he's not an honest person, and he's also someone who is deeply selfish, allowing you to waste savings when he knew he didn't have any income to save his own pride is looking out for him and him alone. No thought of you and your son.
OK, spend two months looking for an ideal job, but after that he should have been looking elsewhere, even signing on and looking for something. The fact he hasn't extended his search in all that time and doesn't even have a broader CV for retail and didn't even bother with it speaks the loudest to me.
I've seen financial ruin by a selfish man, and this is what it looks like. My uncle was a selfish gambler (also keep an eye on that, the fact he wasn't working says maybe he was trying to win some money). Without her knowing, he had remortgaged her house twice, her own son, to gamble it all away. He was a smart man, a member of Mensa, had had a job in MI6, but could never hold one down as he only wanted to gamble. He didn't want to work at all.
It left her in financial ruin when she found out and then she tried to take her own life.
You cannot trust this man financially, that's what you know now. You are unmarried too, that leaves you in a vulnerable position.
If you stay with him, make sure you are firmly in control of all finances. He cannot be allowed to take control because you'll never know if he blindsides you again.
I agree with others, tell your mother. It's controlling for him to tell you not to, so he can still be thought of well in her eyes, as opposed to what he really is, and it leaves you vulnerable yet again and with nobody to talk to.
Personally, I think he's been lying before these five months about his work and job. He could be a pathological liar, he could also hate working and just not want to do it, but kept up a pretence.
If he really did lose the data, is it possible he did it on purpose to be fired deliberately?
The main point is though, that he is out for himself and not you as a family. He did all this to save his own face instead of help you financially and didn't even motivate himself to go out searching for work as your savings started to disappear.
Never trust him with finances again, it's all very well when people say you have to forgive and move on, but never allow yourself to be put in a vulnerable position by anyone, and clearly he wouldn't think twice about doing it to you.