Can my nephew go on fishing trip to Canada with me?

In most cases, including the one you pose here, a judgment really cannot be reached without a lot more information. In general, if the parent with placement gives you permission to take her son on her placement days for a pleasurable outing, that ought to be enough. Do you know why the father objects? is he just being difficult, or is there some specific problem he sees in the boy spending this time with you? If there is no legitimate objection, and if the father does not have impasse-breaking authority, you should be okay. Any advice of this sort is necessarily tentative, because I do not have all the facts.