Most men got feminine posture due to the nature of their job and bad posture. They fail to gain conciousness about their posture throughout their daily activities. The forward head syndrome is caused by poor standing or sitting posture or trying to slouch as a result of being too tall. This makes the chest and front neck muscles becomes tight to pull the neck forward, because the muscles at the back of the neck and upper back are very weak to oppose the drag.
Drooping deltoids could be caused by compressed blood vessels around the collar bone, when muscles press on the them and get choked up by the collar bone. The blood vessels run from the neck, through the collar bone and down to the arm (See TOS in Wikipedia). Visit a Physioterapist if symptoms of TOS persist after weeks of exercise.

Weakened back muscle is as a result of weakness and reduction of the infraspinatus, rhomboids, teres major and minor and rear deltoids muscles of the upper back, and also, loss of the serratus anterior which can cause scapula winging. The infraspinatus, rhomboids, teres major and minor and rear deltoids muscles surround the scapula and are very important for shoulder stabilization. Building the Serratus Anterior muscles, the Infraspinatus and the Latissimus Dorsi will help strengthen the back.
See video for treatment of forward head posture and drooping shoulder
https://www.youtube.com/watch?v=IeKnHtGysBc&itct=CBcQpDAYDCITCJib0If4wMECFQRAywodwYYAcVIkdHJlYXQgZHJvb3Bpbmcgc2hvdWxkZXIgZm9yd2FyZCBoZWFk&client=mv-google&hl=en&gl=US

HOW TO BUILD YOUR SERRATUS ANTERIOR MUSCLES
https://www.youtube.com/watch?v=URhP3tUogDw&itct=CDIQpDAYAyITCLad_v-FwcECFYuzywod9kYAiVIXYnVpbGQgc2VycmF0dXMgYW50ZXJpb3I%3D&gl=US&client=mv-google&hl=en

Guys who are just anxious to build big chests do all the routines, not knowing the importance of building the back. When you build your chest and Latissimus Dorsi without working the upper and middle back muscles, the tightened chest and lats muscles pull the Trapezius muscle forward, causing your delts to become drooped. This can be corrected by working the back muscles and doing some dumb bell reverse fly - For weight lifters only (See Youtube).

BUILD AND STRENGHTEN YOUR BACK MUSCLES

There are three exercises that need to become staples in your program if you want to build a thick, muscular back and they are chin ups, rows and deadlifts. Everything that isn’t a variation of one of those exercises is essentially a waste of time for anyone looking to add tons of mass as rapidly as possible.
Starting from the top, we have the traps which go from the neck all the way down to the mid back. These can be worked most effectively with dead lifts, shrugs and hang cleans (See Youtube).
Deadlifts should be done for sets of 1-5 reps unless your technique is perfect and you are an intermediate to advanced lifter. In this case you can experiment with higher reps. Just be sure that you can maintain the stiffness in your abs and spine throughout the set. Shrugs are best kept at 6-15 reps and cleans should usually be done for six reps or less.

Next are the smaller muscles around the shoulder blade area which are the infraspinatus, rhomboids, teres major and minor and rear deltoids. These muscles work during all forms of rows. If you are really looking to a bang for your buck and to work as many muscles as possible then stick with big compound rowing exercises like dumbbell, barbell and bodyweight rows (See Youtube). These will work most muscles in your back.
If you are more advanced and want to isolate and directly target some of these smaller muscles you can do that with face pulls, bent over lateral raises, scare crows and external rotations. These smaller isolation type exercises are not necessary but can help prevent muscle imbalances and can put the finishing touches on a well muscled back. Unlike other isolation exercises such as leg extensions and concentration curls, these exercises actually do serve some purpose and are effective and building size and strength and preventing injury. When utilizing some of these smaller isolation exercises be sure to keep the reps in the 8-12 range as that is what seems to work best for these muscles.
Finally, diet gives overall result. Eat up and don't starve, unless your fasting. Avoid sugar and take in lot of protein to maximize gains.