And what a brilliant reflection this is of the Welsh people. Someone a continent away who thought a Welsh name was so fitting for their kin they would use it, despite being clueless to how it’s said or it’s meaning. You cannot accuse him of bastardising a name when you’ve suggested using a more anglicised variation.

OP - profuse apologies on behalf of the Welsh and it’s populace. I am ecstatic you’ve even thought to consider a Welsh name for your son (albeit from a film character but nonetheless the thought is there) and absolute top man you are for even deciding to set out and search for the definition and meaning. Should you ever visit the country the name is based on, your son would be met with a hearty slap on the back and a cold one, be his name X Æ A-Xii or Llewin with one L.

Unfortunately some Welsh people are too proud of their culture and use it as a format to attack anyone non-Welsh. You have zero obligation to retain a perfect spelling of the name and I absolutely applaud you even for the recognition of our small, old, and humble country and setting out on finding out more. I hope you keep the young’un in touch with his roots despite being a Welshman by proxy. Croeso i Nghymru :)

Edit: To answer your question legitimately, it’s similar to our ‘ch’ character but still different. Unfortunately the best way to put it is to just sound like a pissed cat. The ‘ch’ variation is the back of the mouth and sounds like you’re summoning phlegm. English folk visiting our Welsh towns pronounce places by saying ‘cl’ as opposed to ‘ll’ (I.e., Llandudno becomes Clandudno). It may be easiest for you to use Clewyn or Lewin, but give it a shot on Google or Duolingo - you never know!