Ya know, you don't have to do what your Ped says all the time. I don't mean that in an offensive manner, just matter-of-fact. You know what your baby is ready for and when better than anybody else. When it comes to matters like this, where doing things your way or the Dr's way won't affect his health one way or another, stick with what's working for you. You can gradually try a cup here and there, but don't think that you must have the baby off a bottle completely by 15 months. That's just bull honky.
Additionally, it's customary, unless your child is small or underdeveloped, to start the child on milk around 1 year. Hey -- it's a LOT cheaper than formula, ya know?
Mine's 10 months old, is still on formula, with a wee bit of diluted juice maybe 2x a month, water whenever she wants (in a sippy or regular cup), and she's eating all stages of baby food 1-3, some toddler food (conveniently bite-sized) including crackers and fruit puffs, and some safe table foods. She's extremely picky about which sippy she likes. We've tried 4 different ones so far. She likes her Nuby the best (most like a bottle). We use Avent bottles, but she hates the sippy that works with the bottles. She prefers an actual cup to a sippy. Maybe your baby would like a cup -- it's a MESS, but it's not a bottle.
Bottom line? It's up to you when you switch your child over. Hey, technically, you could still be breastfeeding him (if you were going that route) and you Dr wouldn't have a thing to say about switching off the boob, eh?
Good Luck