So the question is, when you hold your fingers straight, as if you were showing someone that you have 5 fingers on your hand, is the little finger bent in toward your ring finger or is it straight like the other fingers? The reason I ask is because some folks have a little finger that is bent in toward the others when held straight and I do not believe there is much you can do in this instance. However, if it is straight like your other fingers, then you can re-train yourself to keep that finger straight while playing.
For most folks, when you bend (curl) your fingers toward the fingerboard while playing, all of your fingers tend to bend toward each other (as if you were holding your palm toward your face and looking at the fingernails on that hand). You will notice that your index finger and little finger both point toward the center of your palm. This common physical trait makes it difficult (but not impossible) to get any decent fretboard stretch when playing. The trick is to keep your fingers straight as possible when having to stretch them apart while playing. For example, play an Fmaj7 this way: 1,x,x,5,5,5 with your index finger playing the 1st fret of the 6th string and your little finger playing the 5th fret of the 3rd, 2nd and 1st string. The only way to do this is to allow your fingers to stay straight as possible to get that 5 fret stretch on opposite sides of the fingerboard.
This is a common technique taught to classical guitar players so that they can reach difficult stretches on the fingerboard. The problem is that your left wrist tends to be hyper-extended. However, the same technique can be used in less difficult stretches while allowing your wrist to stay in a more relaxed straightened position.
Again, when you hold your fingers straight, if your little finger is bent hard toward the other fingers, then you may be SOL.