Get the thought of will it be easier to buy a conventional uke in the future out of your head. That is not a valid argument. The best advice is to have him play the way that is most comfortable to him. I have tried to play righty and just can't do it. I am a lefty and playing lefty always seemed the most natural for me. It's funny because I never even knew a chord chart for lefties even existed till just a moment ago. And honestly, when I looked at it, it just confused me. Grab your uke, hold it and go look in the mirror. That reflection you see is playing left handed. They are making the same chords shapes as you, the G strings are both closest to your head and the A string is closest to your feet. That is how I describe it and show righties what chord I am playing if they ask me. When they think of it as a mirror image, then things get so much easier. But don't confuse the issue.
As a lefty, I can look at any online lesson or tabs and read them. There are no special lefty online lessons or tabs. That argument is out the window too. I think righties tend to overthink playing lefty. But it really is simple as long as it feels natural for the player. Along with it being fun, feeling natural is the most important part in my opinion.
As far as buying instruments, true, I can't just walk into a shop and play. But I don't have any shops around me that sell good ukes so that is not a big deal. MGM strung my Flea up lefty for me. Nalu strung my tenor up lefty for me. And I have a custom Moore Bettah that everyone wants to see but can't play because it is upside down to them (so they know what it feels like for me). Since I usually have a uke on me, it's not really a big deal. But all my ukes but 1 are conventional ukes just restrung lefty.
So, if he decides to play righty, cool. If he feels more natural lefty then that's great too. Let him be an individual and play the way that feels best for him. It will make it that much more fun for the two of you to play together.