I can usually make all the different times when I book DME but I'm doing it through DVC, not the regular Disney site. That allows me to book person by person.
As long as your dh is on the resort reservation, and he is listed as having that DME reservation, he can arrive any day at all..no need to worry about that trip. Just give him his travel voucher to hold and then show when he gets to DME area. If he loses it, he can get another one made up, with proof of ID, at the DME counter at MCO.
For your dh's departure, you can take care of that right at the resort. Or, he can give DME the return info when he is at MCO. Just be sure DME knows of his early departure no less than24 hrs prior to his departure date.