Can man boobs always go away through diet or is there a danger of the skin remaining after weight loss?

Yes, and yes. If you simply have excess breast tissue from being overweight, then yes, your breast tissue will shrink as you lose pounds. You will have some skin contraction but how much skin you have remaining is a factor of how much breast enlargement you have and for how long. It is not uncommon for men to have this excess skin removed after massive weight loss, especially after gastric bypass, etc.