Best split the amount of metacam in two and give twice daily. guinea pigs have a very fast metabolism, so in most cases, splitting works better. Have you been given dog or cat metacam? Dog metacam is three times stronger than cat metacam. Both dosages would be within the acceptable range; the cat one right at the bottom and the dog one in the middle to lower range.
You can wipe him with a damp rag but if a guinea pig is still well enough, it will clean itself up. Some piggies can be very messy. Best have a towel underneath that can soak up the extra!
it often helps to start feeding with a bit of syringe feed, then see whether that triggers his normal appetite and if necessary finishing with more feed. It is a very encouraging sign that he is still interested in food and is eating on his own; in that case, we are talking about topping up rather than syringe feeding fully. You just give him as much feed as he wants in one session. Keep an eye on his weight to adjust the reuqired number of feeding sessions accordingly. You will get a feel for it after a while.
All the best!