I've had the same experience before when I wanted to bring new blood into my colony and i purchased a new male. He was a little small when I got him, but I was certain he was sexually mature. However it just took him longer than usual to become well "equipped"... I had him in the tub with a girl for 3 months before I saw any signs of pregnancy, and a week later I had a litter of 16 pinkies, which is an abnormally large litter, especially for a first timer. Had to give some off to a wet-nurse.

My point to this is, you may have a dud, or you may have a slow grower. You could give him another month while you wait for your younger male to mature, otherwise you may just have 2 more months to wait if you just rely on the youngin.