put a little rice cereal in his bottle. not too much though, you dont want to make it too thick. Maybe a tablespoon full
I started giving my son cereal at 3mths. He is a very big boy for his age and would eat every hour or two so I started the cereal in the evening and it helped him sleep a little longer. I just made it not too thick for him to swallow. He is now 4 mths old and is starting fruit. Just make sure you start out with rice cereal. Make just about a tablespoon or so up at first. I told my doc I was doing this and he said it was fine as long as I don't introduce too many foods at one time. They catch on real quick on the swallowing. I also did it with my older son. Good luck!!
Usually it is best to start a certain type of food for your infant as suggested by your ped. but usually infants start the infant cereal or oatmeal around 5 or 6 months of age. I started my boys with it around 5 and a half months and they loved it. You can either put just a scoop in their bottle or try to spoon feed them a little at a time. But if you do decide to put the cereal in a bottle... do not try to poke a hole in the nipple because it can become a choking hazard. Before you start the cereal though just double check with your doctor to make sure it is okay to start it. I always double checked with my ped. before starting anything with my kids.