There is no need to withhold food as horses have no gag reflex, thus vomiting is not possible.
So, unless your Vet specifically, for whatever reason, tells you to withhold food, do not, feed as normal.
I never, as in NEVER forcibly exercise any horse after gelding...what on earth would be the reason???
Consider this, you are sitting in your bed, minding your own business, after minor surgery , when in comes some cheery moron who has absolutely nothing wrong with her, and cheerily demands that you get up and run around the hospital corridors with her for ten minutes.
If that were me she would qualify for the bed next to me in two seconds flat...let's see her prance around the flipping corridors after that, shall we??
Unless the horse is absolutely rooted to the spot, in which case you need the Vet back, quickly, there is not, nor has there ever been, any reason to lunge or forcibly exercise.
If for some reason your horse has to be confined to a stall then hand walking three or four times a day, allowing lot's of grazing, is more than adequate, but normal turnout, if possible for 24/7 is also more than adequate.
I'm not sure where this myth about lunging started, maybe back when horses being gelded was a really big deal and they were done around four years old, so it was a shock to the system??
I've had horses gelded at every age for over 45 years and I can assure, there is NO medical reason to lunge.
I honestly think it started because Vets needed to tell anxious owners there was something that they could do, when in all honesty, if it is done properly, no after treatment other than a good hard look every day, is necessary.
Cold hosing is also a myth and actually inhibits the healing process by shoving water into the wound.
Unless the horse is really swollen (and lots of swelling does take place occasionally and is normal) do not bathe at all, and if the horse is abnormally swollen, again, you need the Vet back.
Now all that is worst case scenario so don't panic.
You need nothing but your Vets phone number and pain relief for the horse (insist on it, BTW some Vets do not give it as standard) and valium for yourself.
He will be fine.
You are absolutely doing the right thing!!!