I was going to post individually to others, but some have already implied or directly given an answer! So a general reply will suffice.

Now that I have no foreskin, my penis tends to 'stand away' from my body; it protrudes, as if semi-erect. Depending upon the size and shape of the toilet seat (and, by golly, they
do vary!), when I sit I'm in danger of the flow shooting forward, under the seat but above the rim of the bowl. Clearing up the spill is not pleasant; so now I habitually hold my penis down, regardless.
When I had a retractable foreskin, it was only necessary to pull it back sufficiently to expose the meatus. (I recall competitions when I was a boy!)
All in all, making a mess is NOT acceptable. If a foreskin balloons, then suddenly spurts (as does happen), he should retract far enough to prevent it or, if he can't, he should sit, use a urinal or 'go' outside.
To the OP: I suggest you teach him to
habitually push his penis down - show him how, if needs be. That way, even if he never retracts his foreskin when urinating, he will not make a mess. A circumcised penis can spray, too, by the way - sometimes simply because it has no foreskin!
Oh, and please keep in mind that fresh urine is sterile. Never worry if a little remains under his foreskin: his next pee will wash it out. Once he can fully retract, cleaning is a doddle!
Christopher