DOn't mean to be a bearer of bad news or anything but I haven't heard of anything such - yes there is a stipulation that forbids a PR Holder from overstaying outside of Canada for over a year or something but nothing that stops them from traveling back and forth...my husband used to visit me once a year ( (but he's a citizen) and my parent-in-laws (PR Holders) are always flying to and back from Canada - a matter in fact last time they overstayed in Pakistan by a few months but still they got back w/o any problem...
On another note - you don't know exactly what your husband is going through in Canada - maybe he just can't afford the trip or perhaps he has gotten really frustrated with the whole process - in either case i suggest to you as someone in the same boat - just be supportive and understanding of the situation - rather than revolting (unless of course you don't trust him or something on those lines) just be ultra sweet and agree with his decisions - coz at the end of the day when you chose to get married to a Canadian PR Holder u must have known about this brutal process and the effects it would have on your relationship. This process is basically a test of your relationship from what I have seen and you got to be down-right Rock Solid and have full faith in eachother...
P.S. There were various times when my husband would back out of visits and I would wine and cry but then I would just be normal and ULTRA SWEET - That usually got him to me right away - trust me killing with kindness is the best you can do