Nope, no need to cut him back. It sounds like you're doing the right thing by feeding him more
You need to feed an amount to suit your individual dog, and the best guide to that is how he looks - if you're seeing ribs, then he's clearly not consuming more than he needs to.
It's worth remembering though, that the guideline on the packet is just that - a general guide. Dogs are just like people in that some of them run on the smell of an oily rag, and others seem to be bottomless pits. Also like people, their metabolisms may slow down with age - so Mega may not *always* need to eat quite that much. You'll know, of course, if that ever becomes the case, because he'll start to gain too much weight
The bottom line on ALL foods is that you need to feed according to your dog's needs. If the dog is constantly hungry and is thin, then it needs more food (or occassionally a better quality food). If it is too fat, feed less. And of course, perfect weight = don't change a thing.