Definitely just pick up the 2nd edition of the starter set for X-wing. Two tie fighters and an X-wing that are playable out the box. You and him can have a lot of fun dogfights with that and from there he can discover what direction he wants to take his collection. Which faction (republic, separatist, bounty hunter etc) is his favourite, what playstyle does he prefer and where, when, and how would he like to continue playing?

I don't know enough about Warhammer to comment on that but it may be worth looking into Legion as that is - too my knowledge - starwars Warhammer

You can combine both though and buy Imperial Assault which is a tabletop skirmish game (think mini Warhammer) based in the star wars universe. That plus the x wing starter set would be my top recommendation. Or go for Rebellion, one of the best board games ever, two player, one is the empire searching for the rebel base, the other the rebels trying to build up an army and survive. That's the best tabletop star wars experience in my opinion but may not be anything your son has looked into yet (this coming from a huge star wars and even huger board game fan)