My cat poops constantly

Hello,

Can anyone tell me what may be wrong with my cat? His name is Tucker and he
is about 9 months old. He is pooping constantly. Sometimes in the litter
box and sometimes not. He poops all over the bathroom floor, in the
bathtub, in the garbage can, in the laundry basket, in the dryer...., yet
he will also use the cat box. He poops probably 10-15 times a day. I am
constantly cleaning the litter boxes. (I have two cats-brother and sister)
They have no problem sharing the cat boxes. I have had him to the vet with
no success. We have tried Clavamox (a liquid) and are currently trying
Metrondazal, which is a pill. I have found it impossible to make him take
the pill. I've tried hiding it in a treat and in soft cat food. They are
both fixed and for the most part they only get dry catfood. Any suggestions?