I don't think, on the surface, that there's anything wrong here. Teen boys masturbate, and this could be a form of that. Some use pillows, or mattresses, and he might like the feel of the bear.
I don't think you need to get him help - I don't think it's an attraction to stuffed animals or anything. He's not depraved, or anything along those lines. Make sure the door is open for him to talk to you about sex and other important things, and make sure you don't over-react when he does. Masturbation is normal, most teens do it at one point or another, some more frequently than others.
If you want him to be able to talk to you about the big stuff, you have to not freak over the small stuff. And he's 15. If he's otherwise trustworthy, he's allowed some privacy now. Maybe knock before you enter his room. If he was doing this in a common area, talk to him about private times in private places.
I have a few teens myself and I would have to say this is completely normal. I haven't had the misfortune of walking in on my son but he does joke about buying him extra conditioner. It's just a part of life and with all those hormones, what feels good and what is accessible is what they'll use. Dont' worry too much.
Boredom might be part of the answer, and intelligent children are most creative in finding ways to relieve boredom. Help him find an activity that 1)challenges him; 2)occupies a fair amount of his free time; and 3)helps him form some social bonds outside the home - if he's not already doing that.
He could just be masturbating, or have “plushophilia” (basically stuffed toys turn you on) but I wouldn’t worry about either because it doesn’t seem harmful.