I know I also have a tendency to put the babies into bed out of the way that are sleeping up in playpens and then settle the older ones on the main floor. If he was at my house I would need to reverse that so that he goes down last no matter how inconvenient that is so that the others get a 10-15 minute start on nap - ie his shorter nap wake up time will now be closer to the others getting up.

As for the morning nap just adjusting the length of it based on the time he wakes at home is about all you can do with a limit of no later into bed than 9:15 and no later up than 10:00. As for the noon nap if he is last put into bed say closer to 12:30 then he should be getting up the same time as everyone else. He can be in an exersaucer or something similar ie confined while you deal with putting the older kids into bed then go and put him into bed.

I have a baby on two naps right now that needs a bottle before nap so that is how my afternoon goes in that the older kids go down and then I settle to feed the baby and put him into bed which gives me about an hour and a half till naptime is over. The older ones get up, diapers changed, etc. and sometimes start them on snack. Usually our noise of all this helps wake the baby and then he comes down and joins the group.

Then he is in his highchair eating snack while the older ones are done and getting shoes on for going outside. Then they go out (from kitchen door) and then baby is done and I can bring him out to the exersaucer as he isnt' walking nor will parents provide shoes for him.