TTC #3 but with different man in my life

I separated from my husband a year ago because he was an ass, couple of months later one of my coworkers who was also a very good friend asked me out and we became a couple, he is an amazing guy super sweet and takes good care of me and loves my 2 daughters from my previous marriage as if they were his, now he wants to marry me and have a baby well there has been many times where we have unprotected sex and recently we been trying and nothing soo he is sad because I already have two kids therefore he says he might not be able to have kids. After how long of trying should he get checked?