IELTS is not mandatory for your spouse.
However, you get points for this, so it's absolutely worth taking the exam. You get 5 points for the FSW calculation (minimum required 67/100), which is 5%, which can be a significant amount. If you're confident you can get 67 points without the 5 points from his IELTS, you can skip it. You only need a CLB 4 to get the 5 points, it's easy.
BUT if you skip it, the next step is to get the CRS score when you enter the EE pool. Here's where you will miss out on a large number of points if you don't both max out your IELTS.
Your spouse can get you a maximum of
10 points for education (Master's degree and above)
20 points for language (CLB 10 and above)
10 point for Canadian work ex (5 years)
So it's the difference between a 400 and a 440 on your CRS score. The work ex is unlikely, unless you're already in Canada, so you're likely looking at a maximum of 30 points added to whatever you score, if he can max out his IELTS and has a Masters.
You can go read about it here
FSW Requirements -
http://www.cic.gc.ca/english/immigrate/skilled/apply-factors.asp
CRS Score -
http://www.cic.gc.ca/english/express-entry/grid-crs.asp