Dabbo- I think you tell him exactly what you just told us. Keep it simple:

1. Your activities over this shore time period will be unusually difficult.
(the words 'getting through the airport with a baby" strike fear in the heart of anyone these days)
2. Your own personal pain management program generally includes avoiding activities that cause flares...but you will be unable to do that very effectively while on this trip
3. Could he authorize a one time increase in pain meds to help compensate for your unavoidable activities

I think you did a great job of putting it straightforwardly. I think the most important thing is to be reasonably confident and don't ask like it is some sort of special favor. Your activities are going to be different and so will your pain, so you will need to adjust the meds.