Hi,
For long haired piggies it is always important to keep the hair trimmed especially around the back end. Otherwise it will get stained but worse than that is it could be wet and they will be sitting in it.
They sound settled in now and so you can give the haircut a try first. I place everything out on my dining table that I need. Scissors, brush, a comb with a puppy pad and some veggies (to keep them busy lol).
I suggest you have someone there to help you as well especially until you know whether your piggy will sit still or not. One of mine loves getting his hair cut and pampered and will sit still for hours but he is an exception rather than the rule lol.
Start at the back end as this is this most important area in case you run out of time if he starts to fidget. Brush the hair to take any knots out. Then I comb out a section of hair away from the body and cut the hair along the comb. Continue to do this all the way around until the hair is short enough and then tidy up any bits that are missed. Remember practice makes perfect and your piggy does not know if they have had a bowl cut or not. They just need to be clean.
I would never use dry shampoo on a piggy as this would mean chemicals left on the skin. I have not even heard of ones made for piggies. Remember you should only use products made to use on piggies or that other experienced members have used and can guarantee they are safe to use.
I would suggest you try the haircut and see how your piggy goes with that before attempting a bum bath. If he is a boar then this will need more done and possibly more often if he is prone to having a very greasy grease gland. One you become experienced though and get into a routine it becomes second nature.
Let us know how you get on and then we can explain the good old bum bath routine lol xx