I ask you to proceed with caution. Men are not always the most mature beasts on the planet - but you risk a lot if you cancel his PR application and prevent him from coming to Canada . . . for the simple reason that you will then be in the middle of a custody battle. Do you realize you will have to have your husband's consent to bring your son to Canada? If he refuses, you will have to go back to the Phils and fight in court for full custody of your son. It is very likely you will end up having to choose between Canada and your child. Having had to do that myself (under different circumstances), I can tell you it is something you will regret for the rest of your life.
You have to ask yourself - even though you are understandably and justifiably angry - whether this is what you want, and if it is fair to your son to lose either his father or his mother . . . because one is in Canada and one in the Phils. The three year undertaking is simple - and, really, nothing: IF your husband collects social assistance during the first three years he's in Canada, you have to pay it back. That is your only obligation, really. If you allow him - and your child - to come to Canada and the marriage breaks down, then how bad off are you as compared to it breaking down between Canada and the Phils and you, possibly, losing access to your son?
My personal opinion: it's best for the child to have access to both of his parents . . . his Dad and his Mom (together or not), at least in the same country! Unless you want to go back to the Phils to be with your child, you could lose him. It's probably better to decide that maybe there's a chance to patch things up, and get your husband to Canada. Then see how things play out. IMHO, it's a VERY, VERY BAD IDEA to involve Immigration Canada at this juncture just because you want to punish your husband for having an affair. There is a child that you share - and HIS welfare has to be what's most important. Reconsider closing doors for the simple reason of revenge. If you have your son with you - who gives a rat's a$$ what ultimately happens with your cheating husband? Worst case scenario, in 3 years time you're done with him - but you have your son for a lifetime. He may be a cheat - but DON'T let him cheat you out of your son, too!