18 months seems a touch too old to be waking up hungry. I'm not saying that he isn't hungry, but perhaps he isn't eating enough substantial foods to carry him through the night (as opposed to a younger infant who wakes up during the night because their stomach is smaller and can't hold enough to carry them 8 hours).
Also wanted to mention the idea that teething might be waking him up, and the bottle might be his preferred way to get back to sleep... Between 14-18 months, toddlers get their molars in. His normal REM cycle might end around 2:30 and that's just enough consciousness for him to notice his teething-?
This is a tricky situation. At his age, giving him a bottle consistently in the wee-hours could easily become a hardened habit which will be 10x harder to try and break when he is 2 or 3! Actually, habitual waking up for food by toddlers is considered a sleep disorder: http://www.cincinnatichildrens.org/health/info/growth/dia....
Obviously, I don't know what your son's appetite/diet is like. My daughter was (and still is!) a picky eater at 12-18 months, so its easier said than done to try and get your son to eat different/more substantial foods. But perhaps you can introduce more proteins, less carbs, in his evening meal? That should help carry him longer during the night. I made "cheesy egg balls" for my daughter (20 sec microwaved egg yoke with melted shredded cheese), rolled into balls (we don't have eggs allergies and our pedi ok'd her eating eggs after she turned 1). It was quick and healthy, and if it wasn't for eggs, I don't know how she would have gotten any protein...! Also let her scoop the egg and cheese mixture onto unsalted tortia chips & crackers. That was a big hit with her.
Good luck!
P.S. Just want to clarify that our pedi gave her ok for egg YOKE after age 1, not the egg whites.