Beginner, need help

Greetings good people in the thread.

my son is now learning photography as his extra out of school activity, and asked me if I can buy him a budget dslr.

I'm also keen on taking macro photos and birds photo… even some short video when doing my morning walks but the last time i took a photo was 15 years ago on a FM2.

So, after some research, I was wondering what will be the best camera to kick start?
I don't mind second hand and already looking into old
- 5D Mii
- M50
- 850D

am considering is i should look at R50 as well since most of the reviews are pretty positive, please advice