I would choose a character and concentrate on learning him. There are builds online you can use to not get over whelmed at first by the shop.
I play an other MOBA but there is probably a lane that has two players for most of the match, you should concentrate on playing one of those two types of characters so you and your son can maximise play time together, most likely the tank would be the better choice (more forgiving, easier to learn, support character so your son gets to be the hero).