I'd be tempted to suggest a Nintendo Switch for a 6 year old, as it's a good console for younger children and families (a lot of their games are aimed at that sort of market). It's a portable games console and also plugs in to the TV, like a traditional console.
The XBOX One may be a better bet if there are also older kids that may play it, as it has the widest range of games and is still user friendly. If it's not a surprise, it may be worth asking him which one he'd like, as it may be that his friends have a specific console and it may be nice if they can play the same game together.
I was considering suggesting that older consoles like the XBOX 360/PS3 may be a good bet, given how much cheaper they are - but I recently had a 4 and 6 year old around and they couldn't believe how "old" the PS3 which I had set up for them was (the kids were far too savvy!!!).