I have an 8 month old Rotti who's 81lbs. We went to the vet the other day, and she said that he is overweight and needs to lose weight. She is suggesting pretty much to starve him to have him lose the weight. Isn't it bad for puppies to not get the proper nutrients by not feeding him enough... also is he really "overweight"!

You shouldn't have to starve a dog to get them to lose weight - just cut back.
What are you feeding him? What about treats?

We are cutting back his food. He was getting 4 cups a day, he's now getting about2 1/2 cups a day. He get's treats when he gets done going "potty", other than that we have cut those back too. We are using Nutrisource, and are looking to change his food. Any suggestions?

Having such a young dog overweight is very dangerous.
It can not only cause heart problems but it is putting a lot of pressure on their growing joints having to lug all that extra weight around.

You might want to consider looking at dry dog food specifically for overweight or obese dogs.
I can't recommend brands as we have a lot of different ones in Australia but havea look at local pet stores and check the labels.

At 8 months old he should also not be eating food with more than 24% protein content so be careful of food that are over.

And finally he needs to get out for some light exercise.
You can't take him for full on runs because of both his weight and his age but regular short walks (2-3 times a day for 15-20 minutes) would benefit him greatly.