Is he applying inside or outside Canada? If inside go to 1), if outland, to 2)
1) If he is applying from inside Canada it can be a problem because if, for some reason, they refuse to re-admit at the border it when he comes back from the US, his application will be considered withdrawn. You can and should stay in Canada during the whole immigration process and until your admission is either granted or denied when you apply from inside Canada, whether your touristic "visa" is still valid or not by the time they make a decision.
Background: The application used to be possible only from outside Canada. As a lawyer explained it to me, CIC created inland PR spousal applications originally because there would be special circumstances (ex: sick family member who requires some support from the family, or any other special/compassionate needs ) so that the two spouses NEEDED to be in Canada. In the facts, people apply from inside Canada because they wanted to be together, but you still should stay in Canada: By leaving Canada, even for a short time, you demonstrate that you didn't really have the need to be in Canada all the time. So border officers were and still are, to some extent, a little less understanding. =... Which is somewhat normal... :-[
That being said, your husband is from the US so he's from a visa-exempt (for 6 months as you pointed it out) . Coming back from Canada shouldn't be a problem. Basically, in theory it is possible, but I wouldn't advise you two to go anywhere outside Canada until he gets PR if he is applying from inside Canada. And it would sound quite awkward to ask for an extension for a visa given the reasons mentions above.
2) If he's applying from outside Canada (i.e. the US), it is not a pb at all to go to the US and come back. but yes he should get an extension of his visa until you both go back to the US, or you can just cross the border and come back before July, 8 and that would renew his visa (depending how far you are from the border) since he's from a visa-exempt country.
Hope it helps??