[Closed] Space Chicken Frame Size

My son would like a Planet X Space Chicken - those of you who have them can you let me know how they size up please?

He is 15 years old and 5’7.5” tall.

I messaged Planet X and they have said he’s top of the small size and bottom of the medium. (Yep I know from the website).

Obviously he will grow as he’s 15, but would the medium be too big now? (Neither his mum or are are tall so he’s unlikely to become a giant!).

Currently he rides a 54 Felt road bike and a medium Whyte 801.

Small will be best for right now I think, but is the medium huge or okay and give growing room?

Thanks in advance.