For the all the images, the backgrounds are WAY too distracting, I find myself focusing more on the background than I do on the subject. You want to try and a background that either compliments the subject or simple enough as to not draw attention from the subject.
#1: He's squinting a little too much and it's hard to see the eyes. When doing photos where they're sitting or standing like this, if the subject is slouching it looks awkward and not very flattering for the subject. Have them sit up straighter so it looks a little more flattering. Also be concious of things like bracelets, watches, wallets/keys, necklaces, jewelery. When shooting portraits make sure the subject is aware of what they're wearing b/c often they're not aware they still have somet things on that they may not want in the portrait. Straighten rings and necklaces. Also if a subject has a wallet or keys, in their pocket, it'll cause unnatural shapes in their pants. Another thing to be aware of are things that may appear in between the subject and camera like the blurry thing in the bottom of the frame that crosses over his arm. Things like that are distracting.
#2: I like the flowers but the fence is a bit distracting. Also, he's squinting again. Try shooting at the "golden" times of the day (from dawn to about 9am and from about 5pm to sunset) that's when you'll have the softest light and most golden light. It'll typically be easier on the subjects eyes and skin tones.
#3: It's a very natural looking and relaxed pose. But again the background is really distracting. When doing poses like having the subject rest their head on their hand(s) one thing to be aware of is a subject will literally rest their head on their hands which will typically distort their face and cause their face to look squished. When instructing a subject have them do the same thing you're having him do here but tell him that he's not actually going to be resting his head on his hand, he's just going to be gently touching his hand to his cheek so as to not distort his face. In this pic his face isn't too distorted, but it is a very little bit. Another thing to note when using the hands in the photo, seeing the side of the hand is typically more flattering than seeing the back of the hand (his right hand on his cheek is angled good, but his left hand is a little awkward looking).
I think william has made some good and important points. Also when taking portraits try not to leave too much space above the head as it draws your eyes up and away from the subject, so crop in tighter. Another thing, be concious of the arms and legs and try not to cut them off at the edge of the image.
I think that william has said it all but in the meantime in order to save the shots you can crop the shots thus eliminating the distarcting backgrounds though you can;t in any way fix the squinting eyes.
You have had some good comments, particularly regarding the background. The only thing I would add is be careful what time of day (and how sunny) you are taking your pic. Too much sunshine makes people squint and casts unflattering shadows. So the weather and the time of day is important. Cloudy says are best and either early or late in the day.