Hello Dragonfly. I'm sure that this is a very stressful time for you. This will be sort of wishy washy advice, but it's the best that I can do, perhaps someone else will know better.
Basically child support is decided based on each persons income and guidelines published by the gov't and the courts pretty much follow that. A not bad website is
http://divorce-canada.ca/child-support-calculator . You just have to fill in the blanks.
If your son has the children a lot, then he should be making note of that and eventually going for sole custody or joint custody or even 70-30 custody which can all make a difference in the amount of child support that he has to pay. (and in a big way)
If I remember correctly your grandchildren are fairly young and if they go to university or college he could be on the hook for child support until they turn 24. So what ever he gets signed or doesn't get signed now is going to affect his life for a very long time.
Alimony is a different matter and it depends on a lot of factors,including how long that they have lived together, but a key factor here what is signed at the time of separation. A judge is less likely to change this, even when circumstances change. But it could happen.
http://www.attorneygeneral.jus.gov.on.ca/english/family/divorce/support/spousal.asp If your son's ex spouse won't sign the papers than your sons lawyer should be sending off letters to advise her of what course of action that she will face if she doesn't sign. If your son doesn't have a lawyer handling this, then he is just courting problems. Everything may turn out all right, but the odds are against him and any money that he saves in legal fees may be eventually paid out 5 times over in other costs.
Good Luck and take Care. paul m