So presumably he treated this woman he was seeing the same way as he treated you in your first year together - lots of treats on the never never.
It seems to me that if you thought you had enough money to pay for your lifestyle, then he was probably spending the money on someobody else.
As for bad timing?
Is he fucking kidding? He chose the timing of the affair! Yes, it was terrible timing, but that makes it WORSE, not better that he also hid his thieving from you.
How dare he take away your choice to leave a husband who both cheats and steals? How dare he defend his lies as being done to reduce the risk of you leaving?
He needed to keep you in the dark about debt that you could be forced to pay off in order to prevent you from leaving him? That is the way a confidence trickster behaves, not a husband.
Where is the honour, the bravery, the integrity of this man?
He has been lying to you (and essentially stealing from you, and now your son) throughout your entire relationship about huge, massive things that could put you in serious financial, and potentially legal, difficulties. And he's a cheater who is happy to fuck another woman while his wife needs him most.
Also this:
"He also says that now he knows and loves our son his priorities have changed"
if my husband said this to me I would leave him even though he isn't a cheating, thieving, pathetic little manchild who can't sort out his own business.
Before he "knew" your son he didn't love him? And what about you? You weren't a priority until now either?
Seriously, fuck this guy.