N.D.
I listed two websites that offer good information. I wouldn't worry too much about it. Remember, these are only temporary teeth and while no one wants their children to develop cavities, the comfort you are providing him at night outweighs the chances that he might get a caries or two. From what I was reading, most children get a cavity or two before their adult teeth come in anyway whether nighttime feedings happened or not.
In regards to your friend...Dentists can be quick to point out and blame nighttime feeding for cavities but I think that the information listed said only 14% of nighttime feeders get cavities that way. So just because they said that doesn't make it true necessarily. Just keep on lovin him the way you do, start dental care for him if you haven't...like wiping his mouth out with a damp cloth around your finger after feedings and relax... but do check out these sites cause they are both a good read. good luck!
-N.
http://www.babyreference.com/CavitiesBreastfedBottlefed.html
(this one is from the Dr. Sears website. It is addressing nighttime nursing, but I think the advice is good for formula feeding as well)
http://www.askdrsears.com/faq/bf3.asp