My cat isn't pooping every day like he used too HELP?

the past few weeks my cat has been pooping every few days. he used to go every day like clockwork but lately he seems to be having a problem. he's peeing in the litter box normaly but isn't pooping like normal. he seems to be eating and drinking normal maybe eating a bit less food but i don't think it's anything out of the ordinary. I took him to the vet and they said he had alot of poop in him and to give him something called laxatone to help soften his poo or a bit of milk which he won't touch the milk i put the laxatone on his paw so he licks that off him. i've been giving it to him like the vet said but he still seems to go every few days. when he poops it's ALOT of poop like he's constipated and can't hold in any more and it all comes out at once. The last time he vomited some food and before i could pick it up he was in the other room taking a BIG poop on the floor. it was alot of poop too not really sure how old he is but i'm guessing 8-10years, and he's eating iams weight control and weighs 17 pounds, he lost 2 pounds in 2 years. what could be causing his pooping problem? should i take him back to the vet? he won't drink milk or eat tuna or canned food but he is eating his dry food and drinking water. HELP PLEASE!