I only have experience of 2 surgeries so I'm nowhere near as experienced as some of the other members. I've got a few suggestions, but hopefully someone else can jump in soon with more advice.
The main post-op medication will be painkillers, a gut stimulant and antibiotics. The vet will give strong painkillers by injection, but you'll need to continue to give them yourself by syringe. This will probably be Metacam, but I've also had to syringe vetergesic too. Make sure you have a few 1ml needless syringes at home in preparation. Antibiotics (Baytril is the most common) will need to be given for at least a week, perhaps longer, to ward off an infection. The gut stimulant is to keep your piggie's stomach turning over to avoid stasis. If you need any advice on how best to syringe medication then please ask.
It is essential that your guinea starts eating as soon as possible. Offer small amounts of their favourite food to encourage them. It is common for a guinea to not want to eat immediately after an operation, but if this is the case then you will need to syringe-feed too. Critical Care is the best for this, but you can also give baby food or mashed up nuggets. Any questions about this then, again, please ask.
With the weather getting colder it is also imperative that your guinea keeps warm. The GA often reduces their natural body temperature so they need to be kept snug. If Orinoco is an outdoor pig then please bring him inside as soon as he returns from the vet. Fleeces, cavy cozies etc. are great for keeping him snug.
The vet will make sure that he is weeing/pooing normally before he is allowed home, but you need to keep up these checks too. The poos will start off small and misshapen, but you want them to get gradually larger and firmer (like normal) as he starts to eat more. Don't be alarmed if you see blood in the cage, or in his urine, this is completely normal after a bladder operation. Just keep an eye on the amount - again, this should reduce after a day or two.
That's everything I can think of at the moment. The main things are to keep him comfortable, to check on him regularly, and to encourage him to eat and drink again. The wound from the surgery doesn't usually require any attention as the stitches are usually on the inside. A quick check every day to look for signs of inflammation or a possible infection is all that will be needed (unless your vet advises otherwise).
If anything concerns you then please ring the vet straight away. Guineas can go downhill very quickly, and a GA and operation puts a big strain on them. It is better to be safe than sorry.
If you have any other specific questions then please feel free to post on here and someone will get back to you as soon as possible.
I hope that helps.
Sending lots of healing vibes for your little man.
