Most Helpful Girl
There's probably a perfectly reasonable explanation for this, and I do have a theory lol.
Stickers, like emojis or Bitmoji or meme stickers, all those types of custom keyboard things that can be sent through iMessage or snapchat, SOME people see them as childish. I know a few couples who use them ironically and unironically to be cute or annoying with one another, but it's most commonly used to be funny or annoy friends.
I genuinely don't think you have anything to worry about if he "isn't sending you stickers" but try sending him stickers and see if he responds positively. It may sound childish but digital stickers don't fit into every person's every conversation content. Some people only use them when they're trying to make a point or arguing or trying to be annoying and obnoxious or silly. If your boyfriend and you don't communicate this way then he may just not see stickers fitting into your convo.