I'm sure he will like it... but you will have to take care of it.
He's too young to do that.
And you can't "pet" a fish.
Just get a very low maintenance fish, in a small bowl. Easier to take care of that way. A Guppy. And a water plant. Fish food. A bowl or small cute tank to keep it in.
Guppies, are easy. Don't even need an aquarium filter. But you will need to weekly, change the water out to keep it clean etc.
IF you get a big fish tank... be prepared to upkeep it. I find smaller tanks or cute fish bowls, much easier. And you can place it anywhere. Or move it around more easily.
Then, here is a story: My Husband bought himself a Beta fish. He wanted his own, pet fish. And he got a small tank to keep it in, and a plant and fish food and cute gravel for the tank.
It was so cute. And it would swim to whatever side of the fish bowl you were looking at him from, and he'd "look" at you. Then it died.
Because, even HE... did not take care of it, as he should have.
His friend who has a Beta told him "you don't have to feed them everyday... and you don't have to change the water every week." So he didn't.
Good grief.
Two men, talking about their pet Beta fish... and their "expert" opinion is you didn't have to feed the darn fish, everyday nor change the water every week.
Then he tells his friend that his Beta died. And his friend goes "yah, mine didn't last long either... so I got another one...."
Good grief.
Pet Fish care. Is essential.
Even if it is just one fish in a small bowl.
Care for it.
My Hubby's fish didn't make it.
Poor thing.
Get your son a cute easy fish.
You BOTH, can enjoy it.
Its fun going to the pet store, with your child and choosing one. I did that with my kids when they were that age and older. Its fun. They loved it.
But so their fish is still alive at least. Unlike my Husband's Beta fish.