How can I know my spouse's Net income until he does his taxes, which won't be for a while? Why can't I file my own taxes without that information?

You can use his income amount from box 14 of his T4s, if available.
You can make an estimate, based on sources such as his 2015 Return, or from 2016 pay stubs.
Or you can provide a high-side estimate.
Or you can wait until,he has competed his return.

Any way, do not enter zero unless you know it is zero.