Given the type of knife he's hiding under his pillow, I'd definitely ask him about why he's got it there. It's one thing to keep a prized possession close, but taking a knife out of the kitchen to keep under his pillow makes me think he's scared of something. Ten years old is a little old to randomly become afraid of the dark, but a scary story or movie could trigger it. You definitely want to know whether he's scared of someone hurting him, which would be a significant red flag. Also, gently him that an unsheathed knife under his pillow is not safe, and work with him to find another arrangement that will keep him safe.
...I find kitchen knives under his pillow. He's learned knife safety (boy scouts) but seems to have a hard time resisting the urge to bring them in his room...Many men tell me this is normal....
I'd say this is the best advice. Safety is #1.That’s great he’s interested, but keeping them under his pillow is dangerous.
Ground rules need to be set, then enforced.
You know your child better than anyone, so how you decide to go about this is up to you.
As far as normal, yeah, these kinds of things are common with kids. I wouldn’t get too alarmed, but would certainly guide him into learning how to use and store them safely.