infant poop..

okay so my twins are 6 weeks today. i give them similac pro advance formula or similac pro sensitive. both of their poops are completely different. i know this is probably normal. but one of them, their poop is SO thick and pasty. i don’t know why? he has trouble pooping because it’s so thick. does anyone know how i can fix this or help his poop be kind of more runny? it hurts his little belly and i can tell :(