Hi Vi -
Bless your heart for hanging in there with Max!. Megacolon situations can be so frustrating to deal with.
First, if he is currently constipated, he may very need an enema at this point. I'm sure you are probably familiar with this if you have been dealing with it for six months already. Our medical treatments are more geared toward preventing constipation from occurring, but once it has occurred, an enema is the only way to get him cleaned out.
I have had by far the best success in these cats using lactulose long term to control the firmness of the stool. This is a medication that you can get from your vet or it can also be gotten from a human pharmacy with a prescription from your vet. It is a liquid and I don't think it tastes very good, so I hope Max will take it okay. It can be given 2 - 3 times a day and the dose will need to be fine-tuned so you get soft stools that are able to pass. This will likely need to be used continuously long term to prevent episodes. I have seen many owners think that everything is going okay so they stop the lactulose and pretty soon end up back in the hospital for another episode.
Hydration can also play a big role in managing these cats, so you may to have your vet show you have to give subcutaneous fluids to Max at home, so we can make sure he is not forming harder stools simply as a result of some mild dehydration issues. This is a really easy thing to do, so just let your vet know you are interested in this.
Lastly, megacolons can be dealt with surgically as well. The procedure is to remove the affected part of the large intestine. There are significant risks to this procedure (infection, internal suture breakdown, narrowing of the intestinal tract) and is best performed as a last resort and by someone who has much experience with this surgery. If you decide to pursue this, I would try to have the surgery done by a board certified veterinary surgeon rather than a general practitioner if at all possible.
It sounds like Max probably needs to go get an enema at your vet rather than just starting him on the lactulose as the lactulose will not make the hard stools that already stuck in the colon any softer at this point.
Let me know if there is anything else I can do to help.