Lactulose is something you can fine-tune. It sounds like you need to be giving him more. I would ask the vet for a small syringe so you can measure it exactly, you're going to want to be precise. With lactulose, and measuring it exactly right, you can get any quality of stools you want and with consistent results. I assume you are feeding him canned food and not dry. If he is getting dry food I would switch him to canned.
You can do enemas at home. I know they say it is better to do it in the vet's office because it is a mess and cat's don't like the procedure, but you CAN do it at home. My aunt (a very stylish woman) gave her cat an enema twice a week for years. She and my uncle took him to the garage, spread the Wall Street Journal on the car, and gave it there.
There is a medication that increases intestinal motility. It is called Propulsid (cisapride). I would try that rather than put him down.http://www.marvistavet.com/html/cisapride.html
In addition, giving sub-q fluids can help these cats, and this is something else you can do at home. I have a cat I am giving fluids to twice a day for another condition, it is not that much trouble and takes very little time. If you want links on that, please let me know.
There is a technique that you can learn if you cat is fairly calm. You can learn to manually massage the colon to move feces along toward the exit. It is especially easy if he is not overweight. You simply cup his abdomen with your hand in a lobster claw and reach up on both sides with your fingertips and squeeze. You can learn to feel the colon (it will probably feel "harder" than you expect) and by squeezing you can actually break the feces into sections and help it move toward the exit. If you see some in his anus that is not coming out, you may be able to get it out by putting on a latex glove, dipping your finger in vaseline and reaching in and sweeping it out. Some people would cringe at the idea, it really isn't that bad.
They can do surgery for megacolon. What they do is simply remove the section of colon that has enlarged. Afterward, your cat will probably need to go to the litterbox more often but he will not block like he has been doing.
I hope something in this message helps.