With regards to how he can afford a "custody battle" when he's bitching about $400 a month in CS...first of all, I would imagine that it's the "principle" of the thing...NOT being "told" what to do as HRUs suggested. The second is simple arithmetic in that it's probably only costing him a few hundred dollars to "threaten" you into saving him $2,400 a year (the amount of the increase). Of course, once he finds out that it's going to cost a hell of a lot more than what he's paying and the odds are slim of him getting his "way", he'll back down.
Honestly, I wouldn't stress his threats, but if he does it again...go with the flow. Let him know that if he wants his son more, that is "wonderful"...of course son will need to have his own room and own bedroom furniture and Dad will need to start buying clothes...at least a half dozen shirts, jeans, undershorts, socks, shoes, toiletries and if son is involved in any activities, Dad will have to start providing transportation and sharing in the cost associated and so on and so forth.