X-Wing is really kid friendly I have taught kids of both sexes from age 5+ and only ever had one that failed to pick up the rules and none have not enjoyed it.
For purchases, does your son have a favourite time period/faction/ship/character? That would definitely inform the buying. At least one core set is required, of the two available the newest (TFA) is a better first purchase because it contains newer rules and the damage deck has been updated from the first. As Flinty said a second core set is a worthwhile purchase for the extra dice (Getting extra dice is really a must, five of each type is minimum), damage deck and rulers as well as the ships and pilots if buying the one you didn't before.
But if you are playing for your son then definitely get his favourite ships as early as possible to keep interest up!
As for general tips with playing with kids some things I found;
Start off small and work up, play the mission in the core box a few times before adding in new ships/upgrades, nearly all of the latter ships add new mechanics to the game and these need to be introduced over a few games.
Keep the movement reference sheets for the individual ships, it really helps if they can see every move a ship can do.
During the planning phase get them to hold the dial so that the appeature is facing the same way as their ship is orientated from them (so if the ship is facing accross the table from their left to right, get them to hold the dial at 90degree to the right). This really helps with being able to visualise and plan properly with out flyinf off the board because they got confused! I know quite a few adults who could do with learning this...
Keep the game space as clean as possible, x-wing has loads of little tokens and chits and paraphenalia which tends to clutter up the table, keeping it cleaned up go along way to making for a smooth game.
Work through the steps audibly (we need to set dials, we need to move that ship, it needs to take an action etc.).
Finally here are the resources;
Learn to play;
https://images-cdn.fantasyflightgames.com/filer_public/fb/b4/fbb4f847-b3f8-4fcd-8724-cb03f67c26b5/swx36_learntoplay.pdf
Rules;
https://images-cdn.fantasyflightgames.com/filer_public/40/b4/40b44d5f-7a06-406c-ae6e-183c5297e796/swx36_rulesreference.pdf
FAQ;
https://images-cdn.fantasyflightgames.com/filer_public/98/36/9836c96f-69ee-4d31-9f77-d4d071b5fd95/x-wing_faq_v441lowres.pdf
Running demos;
https://images-cdn.fantasyflightgames.com/filer_public/9e/9f/9e9ff6e3-09c2-4d57-88e1-50e147c53b95/swx-demoguide.pdf
My prefered squad builder;
http://geordanr.github.io/xwing/?f=Rebel%20Alliance&d=v4!s!&sn=Unnamed%20Squadron&obs=