It's a bit difficult to give concrete advice without knowing a lot more, but here are some thoughts.
Does the action with the sidekick in any way impact on the MC's movements? If not, does it actually matter that it happened first in time? Would the world/plot still be understood if readers believed the two events happened at about the same time? If so, don't sweat it.
I can understand why you want to start with the MC, as it's something I always try and do, but your "to move the action up" makes it sound as if the MC's introduction isn't itself very entertaining or hooky, so you feel you've got to get something else in there quickly to keep the reader on board. If the MC's scenes aren't powerful enough, then perhaps you need to re-think starting with him, or at least starting with him where you do. Is anything gained by seeing him leaving school, returning home and then leaving home? Could you delete those scenes without loss and start his scenes somewhere else so that he's doing something more interesting/dramatic? Any chance of such scenes coming before his leaving school, so they're happening before the sidekick's scenes?
If you still want to intercut the MC's scenes with the sidekick's, then I'd advise against doing it after only one paragraph of the MC -- that kind of jump-cutting is likely to be too fast (unless it's a very long paragraph!). How long would the first scene be if you cut it after the MC arrives home? Have we learned enough about him to want to know more, so we'll tolerate a change of scene and POV? And how long is the sidekick's scene? If it's very much longer than the MC's it's going to feel a bit lopsided. As a rough rule of thumb, I think in fantasy you need more time with the MC initially, to settle us into his POV, than perhaps you would with SF, which can usually move at a faster pace.
As for how to signal something is happening earlier, the quickest and easiest way is to preface each scene with a date and time, though that only works if both date and time are understandable, eg in a fantasy something like "The Quoralth days of Langorn" won't be of any help in knowing what comes first! Alternatively, start the sidekick's scene as a discrete chapter and head it "Two days earlier" or whatever.
