Best time to change to do a planned infusion set change, is before a meal this helps bring any problems to your attention a lot quicker due to having to bolus for food..
If he's at school, either before breakfast or before his tea, some depends on what arrangements you've have about dealing with problems while he's in school!
Sounds like you all getting on with it, it's tiring to start with; just so much testing ect, but it does settle down..
My first cartridge and full infusion set change, took me absolute ages to do, as I read the instruction twice before starting, the re-read the again every stage of the way... Now I do a couple infusion change in a flash, well not quite as I have to wait for the piston to return, which holds the process up a smidge lol