
Shampoo depends on the dog - if he's got sensitive skin, go for a hypoallergenic shampoo (puppy shampoos are usually hypo, or you can adult hypo stuff), otherwise any decent shampoo will do - I use Tropiclean and Proline shampoos, although they aren't widely available (I'm a groomer, so I need bigger bottles!!). I used to use Johnson's and Bob Martin shampoos, BM I particularly liked.
Make sure you brush him at least every other day (every day is best), and always, always make sure you've got any and all knots and tangles out before he has a bath - mats in wheatens are hell to get out, as the fur is so soft - mats very easily if you don't keep on top of it. In general though, you can leave the bathing to the groomer - wheatens usually need regular trips, unless you learn the cliping and so on yourself.