I also think it matters whether you've already decided on divorce or not. For me, that would affect how fast/slow I was willing to move in some directions - no point in pulling off the band aid slowly unless you have to, right?
The childcare issue is short term but as DS ages he'll need you for lots of other stuff in the after school & evening times like sports, friends, events at school (book fair, math night, family game night), etc. Would the gov't job also afford you more emotional time with him that he'll need in the absence of his mom?
While you might take a significant hit in income over the first couple of years, would you realize it's impact so much if you are sharing expenses at your brother's vs. maintaining your home? (will your overall expenses be lower even with the added childcare?)