One thought might be to consider placing the artificial light source on the side on which the sunlight is coming in from...in shots 1 and 2, the natural sunlight is coming in from camera left, but the flash is coming camera right, so he has a nose shadow on the wrong side, which makes the viewer aware that the shots are "lighted" or "lit". It's a preference thing...some viewers might prefer the look of flash-lighted shots....others will prefer using the flash to more closely mimic the natural light that was present, by keeping the light all coming from the same basic direction. I do like shot #1, and my first thought was "good flash exposure".
Pose #2 makes him look absolutely horrible...his shoulders are all "peaked", and he look like a total whimp....contrast that shoulder line with the one in #3 and #4. I like shot #4 of him standing against the brick wall; that flat light could be tweaked pretty majorly in post, and the shot made to really sing. Thanks for showing us these shots. Since this is your very first time at this, you oughtta' be doing great with just a few more shoots under your belt!