What can I do about my weak chin/jawline to improve facial aesthetics? (photo)

If your bite is either adequate or you do not desire to change it, then the combination of a sliding genioplasty with vertical lengthening jaw angle implants could be a good treatment option for you.

Orthognathic surgery is the best solution for you. Orthodontic treatment before surgery is a must which takes about 1 1/2 years.  Invisalign is a good option as well. An orthognathic surgeon will guide you through. 

 Hi, I have performed many facial shaping procedures using dermal fillers, facial implants (cheek, chin), liposuction and/or facelifts for over 30 years.  Following my beauty principles, men look chiseled and handsome with angularity in the cheeks, chin and mandibular angles.  From the photos, your cheeks are flat anteriorly (in the front), the chin is weak as are the mandibular angles and there is excess facial fat in front and below the cheeks.  This combines to create an elongated oval shaped face.  The following procedures would create a more ruggedly handsome face.
Cheek augmentation with cheek implants to add angularity to the cheeks and mid face.  Liposuction can be combined to reduce any excess fat to further shape the cheeks and face.
Chin augmentation, using a chin implant, to add projection to the chin creating harmony and balance to the lower face.  I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut bone segments).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  In my opinion, you are a good candidate for chin implant surgery.  
Augmentation of the mandibular angles using a dermal filler (jaw implants have too many unwanted side effects IMHO) to add volume, angularity and flare to the jaw line.
Hope this helps.