My ASD son is 5, and we have taken him on 8 cruises, all with Carnival. We would like to eventually try Disney but we have had such good luck with Carnival that for now we are sticking with them. There are companies that do special autism group cruises if you are interested in something like that, I think the one in FL is called Autism on the Seas. Our local autism group is trying to put together a group cruise also, you might try to find something like that. It would just give you a little support from other parents who are also dealing with the same types of things.
As for the cruise itself, DS really does well with the routine of dinner in the same place/same time every evening, he loves the shows, and he does well in the play area - especially if he can play video games. You do have to be vigilant and watch them, make them hold your hand, make sure they don't leave the cabin when you are in the bathroom, etc. and I'm sure its not as relaxing as say..an adult only cruise - but thats not our life is it? LOL I do expect it to get harder as he gets older and more is expected from him, but we as with life we will take it as it comes.