Can dogs mate other than when the female is in heat?

I own a male pom approx. 3yrs old and was just given a female. I am not sure how old she is, but i would say is around 1 or 2 yrs old. She has no scars of being fixed or otherwise. My male is steady trying to hump and "get some loving" but she keeps squirming away. I'm assuming because she is not in heat..or she just may not be in the mood.{lol} He's being quite persistent, and she's growing less tolerant. How long is my boy going to have to wait? And should i separate them? He is literally about to burst!
Thank you,
Marisa Perez