In my experience, kids ALWAYS want a cute little pet - kitten, puppy, micro pig, bunny, etc. - and within short order, they get bored with the animal and it's either neglected or the parents wind up with the responsibility. The shelters fill up by the end of January with puppies and kittens as well as more exotic animals like pigs, and in May with bunnies and ducklings. Impulse buys never, ever work out.
My suggestion is this - instead of letting a child jump from "holding a cute animal" to "adopting one for life", start researching. First, evaluate your child: has he shown responsibility for other animals in the past? Does he have a strong track record of caring for anything over a period of time? Does anyone in your family know anything about pigs? Do you know what they eat, what care they require, what the expenses are for food and bedding and veterinary care and shots, what sort of interaction they require? Is your child prepared to give up after school activities to come home right away to care for a pig? Who will care for the pig while he is in school? Is his plan just to put a pig in a pen and leave it there, maybe holding it for 5 minutes a day? Is this a good life for a pig?
Have you spoken to a vet about pigs' care and longevity? Do you have other family pets that you need to factor in? Do you know what the investment would be for proper "accommodations" as well as vaccinations and well visits in addition to illness and injury? Have you checked your town/city ordinances to find out what the regulations are?
My suggestion for any child who wants a pet (unless the parents are taking on all of the responsibility) is to spend time in a shelter, volunteering with homeless animals who are lacking in attention and affection. These animals really need the care, and if a child can't stick with it, then they aren't ready for a pet at home.
A pet is a life-long commitment - higher level animals cannot be re-homed easily. They suffer stress and abandonment. That means cats, dogs, higher level mammals, etc. Turtles and gerbils can be re-homed (if you can find someone interested) but other animals form attachments with their owners and you must assume that you will have this animal for all of its natural life. There's nothing in your post that says you are remotely ready for this - although please do update us if you have extensive pet experience with your son. Then we can be more specific.