Apple ID across 2 devices

I use the same Apple ID on my iPhone and on my son’s iPhone. (My son uses the phone just for apps / internet. He has no SIM.)

Until today, we had our own settings etc. but today, his photo has appeared next to the Apple ID in settings on my phone. The worst thing is, he deleted photos on his phone, we checked and they were still on my phone - all ok. But then, they’d gone, at the same time his photo appeared next to my ID.

How do I get back to (A) us both having our own settings and (B - fingers crossed) me getting those photos back.

any help much appreciated. Thank you.