This may sort itself out by itself, but very often water is the answer, I would make sure it has been boiled and cooled, and you can buy it in bottles now ,for babies .Pureed plums/prunes may help, just a little , and if you have him on rice cereal try oatmeal or baby muesli ...breast fed babies do need some supplementation of water ,make sure that his food is sloppy not stodgy ...good luck