Lieing about finances *long

Say you're in a serious committed 
Relationship. Not married yet but plans to get married in the future.  You are engaged. Living together. & TTC. Your SO has access to all your finances (you have a shared bank account) & he knows everything as far as the financial stuff is concerned. (How much is in your checking & savings accounts, how much money you make..ect.)       He receives a tax refund & you do not because you have to pay in.      His tax return = $3735.xx (you know this because You have access to his email account) which he had put onto a prepaid card, as that was the fastest way to get it. He then takes $1000.00 & puts it into your bank account (remember it's a joint account). & proceeds to tell you that's all he received for a return.. He lies to you. Because you already know otherwise because you read the email (which oddly was in the "trash" when he never deletes anything)      Also for background (this may or may not have anything to do with it): ex wife has been consistently on his case about his taxes & when, or if he has received them.    Should I be concerned about this??    (Also for background we've lived together 8 months now, & I have done the majority of supporting (paying bills, buying groceries, things he & his children need, ect.) because he has had poorly paying jobs, gotten laid off a few times, & I make more than him & am more financially stable) 
Would this bother you? Should I say something? Is this lie something to be concerned about? Is he planning on giving money to his ex? Should this be a problem for me or am I over reacting?? Any thoughts or advice on this would be appreciated. 
Thanks Ladies. 💕💕
Edit: he has his child on the weekends & pays $100/wk child support to the mother. 
Edit: also I know it's weird looking at his "deleted" email... But he was disloyal in the past (chatting with others, nothing physical) & that's why I do it. Yes I chose to stay and fix the relationship but fool me once, shame on you...fool me twice...