When she's going into season, you'll notice that her mood changes most dogs become more clingly, start to lick themselves a lot, her vulva will swell and of course they are going to bleed. A way to check for blood would be light push with a white tissue against her vulva and see if any blood pops out. As soon as you start seeing that she's in heat I would board him because you don't know when she is going to fertile. I would keep them separated for a full 28 days.
When she's not in heat, your boy will have no desire to mate with her since she not in season. so after she fixed they'll be fine.