OP - where are you?
Understandably, emotions are raw right now. I suggest you and dp discuss this like responsible adults when you've both calmed down.
Personally, i don't think his choice to be a SAHD makes him a 'crap' dad or means he's 'shirking his responsibilities'.
He has every right to move on with his life just like his ex - that does not negate either parent from their responsibilities towards their dc.
How he manages this and the actions he takes will determine whether he's guilty of the above.
This would be an ideal time to re-think contact schedules and shared parenting. The DSC are old enough to have a say in this.
He would be in a position to have a more 50/50 arrangement, thereby taking on more responsibility for his dc.
I think it's an ideal way to blend the two families especially with a new sibling on the way 
'Maintenance' would still be a priority if he wants to treat his dc as equally as possible - only difference will be that he pays for his dc 'directly' rather than 'giving it' to his ex.
In my opinion, you and dp need to sort out how you're going to share finances and manage budgets moving forward.
Since you're having a dc together, all income is family money - it does not belong to the wage earner alone - just like a SAHM, your dp is actually a joint wage earner in this arrangement.
The DSC are a part of this family and your dp is entitled to use some of this family money/'his half' of the 'wages', to continue paying ex maintenance until situations change.
Teenagers 'cost' more so his financial responsibilities towards his eldest dc would be changing anyway.