Constipated 4 month old

I started giving my 4 month old son rice cereal a week ago after the doctor gave me the okay. I got a banana apple flavor so of course he’s loving it. But I’ve noticed he hasn’t pooped in a few days now. He pushes but only a few marble sized poops come out. He’s not fussy or anything but as a first time mom I’m worried. Is this okay? Should I be cutting back on the rice cereal? I put 1 scoop in his bottles. I made an appointment with his Ped for Thursday but I thought why not ask some of you momma’s on here until then. Also here is a pic of my little one so you can see he’s a happy baby and also because I’m obsessed with him 😌😍