They're in yr6. The friend joined the school part way through year 5 and has been one of the "crowd" DS1 runs with for a while. DS1 doesn't have "best friends" as such, but has a close knit group of 6-7 boys, which seems usual for boys?
Anyway DS1 seems to like this new friend and he's been to tea at our house once. They live about 2 miles from school out of catchment with at least 2 other primary schools nearer to home (unusual at this school, it's not the kind of school people fight to get into iyswim)
Organising the tea was hard work as mum is never at school - he waits for his older brother (14yo ish) to get off his school bus here and they walk the two miles home together. DS1 kept on though so I gave him a note with my contact no and his friend's mum called me to arrange. Tea was fine, seems like a nice boy, brother came to collect him (also very pleasant) and mum text me thanks.
Over tea, I made some inquiries about why he'd moved school and where they'd lived before etc, but he didn't answer any of my questions, telling me it was a "long story" or he couldn't remember.
Anyway, yesterday DS1 got a Christmas card from him which says "Dear DS1, you've been my best friend, thank you for your support" Which 1) seems like a strange thing for 10 yo to say and 2) is sad that he felt the need for "support".
DS1 first tried to hide the card which naturally made me seek it out and when I asked him about what it might mean / why his friend wrote it I get "dunno"
So, over thinking or cause for concern?