How can I make sure my baby goes back to breastmilk?

So my wife just spent a week in the hospital, and due to the pain amount and type of pain medication she was on, we had to switch our 6 week old to enfamil. Mommy's going to be able to breast feed again in a few days, and we want to switch him back... I gave him a few bottles of her milk we had freezed, and while he takes it just fine, it doesn't seem to satisfy him. On the formula, the most he had ever taken in one sitting was 4 or 5 ounces, and just now he took 4 of Mommy Milk, and another 2.5 of enfamil after...

So should we keep doing what we're doing, by trying to feed primarily on BM, and then 'topping off' with formula? or try to switch him cold-turkey or something? thoughts?