I gave the boys their wormers around 7.30 yesterday morning and, probably because of the large quantity of horse s*** he ate before this, the youngster threw everything back up just after 9am.
Two questions - would the wormers have had a chance to work and, if not, when would it be safe to redo him.

If it were me, I wouldn't - assuming you worm pretty regularly anyhow.     If you're worried it's all gone out again, worm him again in a month or so - it's unlike to make much difference.

Jo