What fish would you recommend for a beginner who wants an aquarium?

Goldfish might be easy to take care of, but they require very large aquariums since they can reach 6~12 inches in length depend on their species. They also eat a lot and produce a lot of waste. So the general rule is that you need minimal 20-gallon for just one fancy goldfish, and 30-gallon for just two. If you get common goldfish, they are even bigger so they require minimal 40-gallon just for one, and 55 gallon for two.

When you mention "for beginner", do you mean "easy" fish?
Most fish species are quite easy to take care of if you do things correctly. Such as getting an ideal size fish tank, filter, heater, air pump, water conditioner, high quality fish food, etc. and get the aquarium properly cycled before get any fish.

I would suggest you to read through our articles of aquarium nitrogen cycle before getting started. It is highly recommended to do a fishless cycling before get any fish. Of course, you need to set everything up before even start the fishless cycle. So I guess you can't give your son a surprise after all. Getting pet fish is not something you can just one day bring them home without any preparation.

As for the actual fish species you want to get for the aquarium, it depends on what kind of aquarium you want to have. Will it be a community aquarium with a lot of fish? Or do you just want a few fish and be done with it?

If it is the latter, you might as well get a minimal 40 gallon fish tank for two fancy goldfish.
If it is the former, you can get a 10 gallon tank for no more than 10 small sized tropical fish of no more than 2" long. Some of the hardy species are, Zebra Danios, Platy, Guppy. But i would not mix them up in the same tank since Zebra Danios are nippy and fast moving, while the other fish are small and have have longfin as perfect target for nippers.

Keep in mind that if you get goldfish, you will not need a heater. To get tropical fish, a heater is a must. You can't mix them up in the same fish tank.

Actually I would recommend you to decide on the size of the fish tank first before decide on what fish to get. Sure you can do it the other way around if the size does not matter to you.