I wouldn't give him a dunking....but you could just dampen the offensive "bit" and that might encourage him to wash himself...that is why cats rarely are as whiffey as dogs.
Better to have a helper whose job is to hold (NOT squeeze) the nape of the neck just as the mother cat does. Have the water at the proper temperature ready in containers (don't run the shower or taps when the cat is in the bathtub). Place the cat in the tub, or sink, and gently wash with cat shampoo, and rinse. It's not as bad as you think, especially with a young cat. But you should really have that person to assist you.
If it's just his bottom end that's smelly, use a very slightly damp sponge without any soap or anything like that on, he will almost certainly start cleaning himself if his rear is wet! please don't bath him, not if he is still very young, he will never trust you again, an older cat would probably get over it.
I have 2 cats and have never had to give them a bath so far. However, I have read this in a cat care book:
You have to hold the cat by the scruff of the neck (just like when you used to lift it when it was a kitten) and shower it with a shower head. You can do it gently, with the water not on at full power, and neither too hot nor too cold. The cat still won't like it, but sometimes unpleasant things have to be done and that's the recommended way of doing it.
perhaps its poorly if its not washing itself, mine are always clean.
is he long haired? i had a persian once and had to clean its eyes and bottom daily with a damp cotton pad and some scissors to cut the poor away!!! it was ok cos did it every day!
try damping your kitten and see if that starts her washing!
I have had five cats over the last thirty or so years, and all except one has kept clean. However the one that didn't (and still doesn't) only has to be out the house five minutes and comes in absolutely filthy. He is too old to bath now, but in the past I have stood him in the bath (making sure the door is shut and plenty of towels at the ready) and then gently ran warm water in the bath so as not to panic him. When I first did this he scratched and struggled like anything but in a short while actually seemed to enjoy it - I think he must have been glad to feel clean. It isn't really any different to washing a rebellious child! Good Luck