Please bathe him in a basin in the bathtub or shower; this prevents him from injuring himself in case he jumps blindly. The sink is unfortunately not a safe place for bathing guinea pigs.
You may not see any or any normal poos if he has hardly eaten over the last day or two, but it is a very encouraging sign that he is lively again.
It takes time for the guts to stabilise and to process food; the output tends to lag one to two days behind. You can help him if you have got a brown paper bag at home (cut through any handles); nutritionally poor fibre is often what guinea pigs with a tummy that is out of order crave.
Or has he got a companion? In that case, you can soak some fresh healthy poos in a little bit of water and syringe that. It is a bit gross, but it contains all the right stuff to re-stock the guts. A probiotic from the pet shop can also help prop up the guts.
Here is our diet guide for a healthy long term diet. Stay off any veg until the poos are looking normal again for a whole day.
Recommendations For A Balanced General Guinea Pig Diet