Ay, yellow poop sa babies is usually normal lang, lalo na kung newborn pa or breastfed siya, it's that mustard-yellow color na parang curry, tama ba? But if it's a different kind of yellow, like greenish or too pale, or kung may ibang signs like fever, vomiting, or he seems uncomfortable, I'd suggest checking with your pediatrician agad to be safe, kasi mas better na i-rule out anything else.