Recall is one of the most important, if not the most important command every dog should know. From my experience, one of the best ways to teach recall is via positive reinforcement, and also starting 'small' and simple, and then slowly increasing the difficulty.
For example, start in a small room with minimal distractions --
1) Clearly verbalize the command you want him to learn (in most casts, the word 'COME', but it's up to you of course) in a neutral manner, in other words, you shouldn't sound like you're asking him to come, but rather, telling him without being too forceful
2) If he does come to you, apply positive reinforcement by immediately praising and fussing over him. You can also throw in a rub/scratch at his favorite spot, or use training treats. You do this because you want him to realize that he did exactly what you commanded (and also that there is a positive outcome for him in doing as he is told).
3) If he doesn't come, just keep trying to get his attention, be it by tapping the floor or raising the pitch of your voice. These are just to get his attention and entice him to come to you.
4) Repeat step 2 over and over again -- if you are doing it right you should see him start to 'learn' the command eventually.
5) Once you feel he has almost mastered the command when you give it to him in the small room, increase the level of difficulty slowly. Try the same thing in a bigger room/area with Harley further away, or perhaps with another person in the vicinity to serve as a potential distraction. Again, rinse and repeat.
6) The key is patience and consistency -- give Harley a couple of these training sessions (maybe 10-20 minutes each time) each day, and if you are doing it right I'm sure he'll be a master at recall within a week.
Hope that helps!