I have a rat, hamster, guinea pigs and a rabbit.
I 'd say firstly a cold water fish is the most basic pet u can get feed it once a day and clean it out a couple of times a year depending on how much daylight the tank gets, more light = more algae (green muck) = more cleaning.
It will develop basic 'looking after a pet' skills but your son will probaby want a touchy feely pet to cuddle and stoke.
So if you are happy to clean large size cage one a week twice a week in summer then get a guinea pig, I have 7 all indoor all adorable.
They would be the perfect companion as they very rarely bite, some are more lazy and some are more active than others but that is hard to judge when you are going to buy them so young their personally only develops once they have settled at their new home. Get a short hair not a long hair as it does not require as much grooming.
I would recommend keeping it indoors if possible as they are safer, get less medical problems such as over heating and flys, will be more hyginic for your son to handle, and most importantly gets a better bonding experience with your son - it is enevitable that a pet living outdoor gets less attention. Guinea pigs make lots of noises which are such a pleasure, such as when they hear food, carrot being chopped etc. Your son will love it.
Try and get a cage which is easy to clean I recommend ones with plastic base with transparent top which clicks on top - this has wire mesh section in top to open and put food in/get pet out. I find the fully wire top cage allows food and hay to fall out all over the floor and is hard to clean, where as the fully plastic cages are easily wipeble..