Give him some exercise and his old diet and he'll probably slim down. It sounds like maybe he had too much food and not enough exercise at your friend's house.

The internet is saying that an American-type budgie should be 25 to 35 grams. English budgies are bigger.

Thank you.

How can I exercise him?

Encouraging him to fly when he's out of the cage with you is a good way.

Ummm... He's clipped. I'm sorry

He can still flap and do some traveling. A clipped bird actually has to work harder to travel the same distance as a fully flighted bird, so he'll get more exercise.

I dont want to sound helpless, but, how do I make him fly or move?

Make them fly to you for treats. Slowly make the distance further until it's the max they can fly when they're clipped. I'm not sure if the treats will make them fatter or not. You have to find a way to encourage them.

Weren't you training him to fly to you, at one point? Might be good to continue that training.

Ohhh that's true. I forgot. I guess it'll have to be individual training