Travelling for a short period, week or two in itself will not be a problem. The problem is if CBSA denies you entry into Canada. Why don't you travel along with him, have all your application documents like the AOR, Medical requests and etc with you and at the boarder tell the officer you have applied to sponsor your husband for PR and as part of it his son needed medical exams and that's why you went to USA and now returning back to Canada. I don't think they will stop you from entering back, but again do this only if you have no other way.