It really depends if your son wants a cuddly, friendly, social pet, or not so much. With hedgehogs, personalities can be molded to be more friendly, but sometimes you just end up with a stubborn hedgehog. I was told good advice when I was young and got my first hedgie;
Every day is a new introduction to your hedgehog, it's like, they are rediscovering you all over again.
With that in mind, go slow with a hedgehog, don't force him/her to be social by keeping him up long hours during the day (they are very nocturnal).
THat is why I worry the hhog might end up not being the best choice... he/she will sleep all day, be somewhat cranky when woken up during the day, will keep looking for a place to hide until they do, then they will go back to sleep! At the same time, the spikes have a minor skin irritant on them (aside from being spikey!) So you might want to handle a hhog first and see if you break out in a rash or are only minorly bothered by it.
But, a friendly and social hedgehog who gets lots of playing time in the evening can be a fascinating and hilarious pet!!!
And yes, they will hunt down and eat all your gross insects and spiders!!!
And it seems that from what was said above, the initial cost of a hedgehog will probably be greater (they cost around $100 dollars, and you need a big cage with a big wheel and toys) but then you also need appropriate bedding, high quality cat food, and fresh fruit and veggies to feed the hedgehog. The only major health concerns are skin problems (mites, quills falling off) and tumors but with good care and frequent cleaning out of the cage/toys, you should have no trouble. But by and large, hedgehogs are NOT high maintenance, and many hhogs actually love being somewhat independent
For example, my Izzie gets fussy if I take her out every evening for play.. it's just not fun for her. But she does like getting out every other evening, because by then she is tired of her house, bored of her wheel, and wants to see what new crap has accumulated on my carpet! :3
I'm not sure about owning a gerbil, so you will have to get advice from gerbil experts on that