I bath my guinea pigs all the time. Sometimes pet stores carry guinea pig shampoo but it is hard to find (I speak from experience) The vet said that tearless baby shampoo is fine. I usually bath my piggies in the bathroom sink, I fill it about a quarter with water and I simpley lower the piggie into the water. Some don't like it, try to make sure that if he decides to jump, or climb out that u have control of where he is going (control pig with one hand, shampoo and rinse with other) You can blow dry them with the dryer set on low , but I don't like to do it that way. I usually towel dry mine.
Good Luck! try not to get any water in his eyes, they don't like that very much