I always get myself in trouble when I respond to these threads but here goes. . .
The nurse was wrong and wasn't even following AAP guidelines which are 6 mos. You do not
need to teach a baby how to eat from a spoon, he will do it on his own when he's ready. . .which will be much later. . .closer to a year old for him probably. It's a developmental skill--and it drives me bonkers when people say you have to 'teach' them how to do it. When you do start, you'll start with a spoon and it'll go from there. I promise.
Rice cereal is essentially empty calories. So no, for him, I wouldn't replace a bottle of formula with rice cereal. It will do very little to help him. If on the other hand you're feeling comfortable feeding him fruits and vegetables, yes, that would probably be better than formula. I would highly recommend starting with something like avocados and then maybe bananas. (or maybe bananas first and then mixing it with avocados). Avocados are super high in calories (which is right up his alley) and have super good for you fats in them. My kids loved the mixture of the two together. HTH