Yes you should wait until 6 months to start other foods than formula/breast milk. To each their own but rice cereal has no nutrition for a baby. So, baby is getting a full tummy but its like theyre receiving empty calories. Maybe try feeding him a little more at a time to see if that works or more frequently. He could be going through a growth spurt. Most of all if he still isnt satisfied maybe see what your doctor suggests.
My son was eating 5-6 ounces at 3 months. You might get negative response to this but I have 2 kids and this is the only thing that worked for my kids. Like the above comment, add a few pinches of cereal. I started mine with regular rice cereal and they were a lot more satisfied. Every baby is different and they say wait til baby is 6months but you have to do what u think is best for ur baby. If ur still unsure, ask ur pediatrician for their opinion.
Try a little bit of oatmeal.. My dr said it was fine to add a little bit.. Plus she got fuller faster..