Playing guitar upside down

My son has suddenly decided to play guitar. He has downloaded an app and is learning from that. He's using one of my small body acoustics. He knows theory and piano to a high standard. He is right handed.

But, he is insisting on playing the guitar left-handed meaning the strings are upside down as well obviously with high E at top.

I can't actually think of any reason why not? I've been playing for so long I can't think of anything obvious other than thumb muting and thumb fretting (which I use a lot). Any obvious blockers if he carries on this way? He's actually gotten quite far in a few weeks.

Edit: Everyone thank you very much for taking the time to comment. I'll share this thread with my son to read.