Unfortunately, I would not agree with putting a seahorse in a tank under 20 gallons at the minimum.
Most of the species that would be practical and not on the difficult to care for list, would be too big for a 14 gallon....they should even outgrow a 20 gal.
How old is your son?
My own experience was that, while she did think they were cool, my own daughter thought that the seahorses were, overall, pretty boring.