Can my boyfriend still file written reports for my daughter, for the year 2015 and 2016, she was claimed both years by someone else or should we wait?

My boyfriend tried claiming my daughter on his taxes for 2015 and 2016 and both times were denied because someone had already claimed her. I want to know if its too late to file a report and how do i go about doing so. Also can we start filing his 2017 tax return now or should we wait, can we inlcude my son he was born august 5th of 2017.