I think you made the change in his diet too abruptly - he was used to a full bowl of food and now he's down to a drastically reduced portion. Imagine yourself being used to 3 meals per day and suddenly eating just one...
You're right to worry about his weight. Obesity in cats can cause health problems just as it can in people so it's wise to change his eating habits while he's still young.
Dry food contains lots of carbs which has the same effect on cats as it does on us. Most cats don't need to be free-fed dry food 24/7. As far as the recommended amount to feed that's on the bag, those are just general guidelines. Each cat's needs are different depending on age, health, activity level.
Sounds like your kitty would benefit from being fed two main meals per day instead of free feeding. Get him a high quality canned food - it's much healthier than a steady diet of dry. Look for a food with no by-products or anything artificial. Feed him a measured amount, maybe start with 2-3 tablespoons. You can sprinkle a few pieces of the dry on it to help him get used to it. You'll probably have to adjust the amount depending on any weight loss or gain that you see. Weight loss takes time, so be patient.
You could also give him a SMALL mid-day snack of just a few pieces of dry - leave it in an automatic feeder if you won't be home. This may help him get used to the new way of feeding and cut down on his begging. When you determine the optimum amount to feed him am and pm, you can eventually cut out the snack, or just give it occasionally.
Exercise will help slim your boy down too. Get some new interactive toys (wand toys with feathers, mice, etc. attached, or a Cat Dancer) and have a couple of play sessions each day. Besides, you want to reward him for being such a good boy about his new diet!