Is he not gaining enough weight that they're concerned about it? I think my first choice would be to pump after he has nursed and give him that pumped milk. We did this with all 3 of our babies - gave them the milk via syringe because we didn't want to deal with nipple confusion. You could also use a supplemental nurser system to simultaneously feed a little formula while you're nursing. Unless there are specific health/weight concerns, I wouldn't be eager to supplement with formula after every feed. Your hospital's lactation consultant might be able to assist.