Did quick calculations on
TaxTips.ca - Canadian tax calculator, assuming you (not your ex) claim your son as a dependent and receive all of UCCB, and you both live in Ontario.
You: 45K + 1200 UCCB minus taxes leaves you 37930, plus the $312/month child support = 41674 (average tax rate = 17.9%)
Ex: 81K minus taxes leaves him 58718, minus the $312/month child support = 54974. (average tax rate = 27.5%)
So your NDI, just based on taxes and CS, is 41674/54974 = 76% of your ex's.
I'm sure there are additional factors that go into that calculation, but still I think that's pretty high, meaning you might not qualify for spousal support.