I'm a lefty beginner, having played for about 7 months. I think a novice lefty actually has an advantage playing righty, because moving smoothly from chord to chord is the first challenge for a beginner, and they are mastering that trick with their dominant hand. Maybe when you get to finger picking and such the advantage evens out, but I'd teach your lefty kid to play righty. It will also be way easier for him to read chord diagrams, etc. but when it comes time to teach him golf, get him a set of lefty clubs. My Dad insisted I try to learn golf right handed, because Ben Hogan was a lefty who golfed righty, and allegedly had an advantage because his dominant hand was his power hand. For this uncoordinated little kid having to learn swing and technique was just too high a hill to climb, so I dropped golf early. But your lefty should thrive with a righty uke!