The starter sets are good for just that, getting into the game with some solid basic decks. The Boba/Luke ones are good, as are the Obi-Wan/Grievous, I think it's a personal preference on characters really.
As for looking for specific characters for deck building, there are no defined sets that focus solely on a part of the Star Wars saga, sure some sets will feature some aspect more than others but in general there is a mix of things in each set. The best thing to do if you are looking for characters you want to build a deck around is to go for the singles market. There are some very good websites with fair prices on singles, especially if you are looking at any sets older than Spark of Hope, depending on where you are in the world. Here in the UK my go-to site is Watto's Card Yard, but I have used a couple of others too. eBay is also good, but you pay a bit more to cover the sellers fees, and of course Facebook buy and sell groups are also solid places to ask around for specific additions to your deck.
I hope you and your son enjoy the game! It really is still such a good game to play, especially on a casual level where you can really go nuts with the deck ideas!