Rice cereal is VERY constipating. I learned that the hard way. We were thickening bottles w/ rice when my dd was only 2 months old as well and she was in even more pain from the reflux and the constipation!!
I’d recommend to stop adding rice cereal. If you must, add oatmeal instead!
If he is indeed constipated, (strains, grunts, cries and pushes out a little pebble) try giving him extra water, or a little juice. If that doesn’t help, then I’d use a glycerin suppository.