Quoting SheldonsMummy :" My son is going to one in 2 weeks.. He has 3 formula bottles a day. one when he wakes up, one when ... [snip!] ... I dont know how to make him eat more if i try force him then he crys. Is he just really not hungry or is something else wrong?"
I have a similar problem with my DD. Her doctor suggested getting a multivitamin or something called Prosobee (?) and it will increase her appetite. Otherwise during growth spurts or illness it is not unusual for their appetites to change and they will eat when they get hungry. I say call his doctor and see what they say but definitely dont try to force him to eat as this may lead to aspiration of food or possible choking.