Whistle training is great But you want him to come with one whistle right? So wen you blow it four times and then he comes, he's learning that you can blow it quite a few times and he will still get rewarded when he eventually comes. I would start in your house or have him on a longer lead and blow, treat, blow, treat quite a few times. Like how we associate good things with the clicker, click treat click treat. Do this for a few minutes a couple times a day so he knows whistle equals treat. Then take it off leash and blow, let him come (because he knows what it means), treat. Eventually, when it is really stuck in his head, you can start alternating your reward. Beagles are smart, he'll pick it up in no time.