I read a book when I was between 9-12, I’m 38 now, I’m not sure if that helps with the timeline, I borrowed it from my grandma but it wasn’t really old, and it was paperback.
There was a little boy, maybe 11, he found a gorilla/orangutan costume (but enough for a grown man) and he started to believe it was talking to him. I remember him holding his hands over candle flames and if I’m remembering correctly, he ended up destroying the costume in the end by throwing it in the fire.
It would be amazing if you can find it, it’s been bothering me for years! Thank you again for your time and I look forward to hearing back!!