I see.
Even if the client is directing you and you don't sell the rights to him, he shouldn't be using the drawings outside of what the original purpose was. Do you have any kind of contract or at least maybe an email discussing the terms?
When I work on commissions for companies, they usually have a contract ready that specifies where and how the art may be used by them and how I can use it. Or at least we discuss it elsewhere. They write down basically where and how they can use the art (maybe even for how long), and I am usually interested if I can sell the art again and if I an put it in my portfolio or basically share it online.
I am not very experienced in the bussiness part, either, but still. Using your art for basically an art
competition, seems... off. You should at least get part of the price. However, if you agreed for the stickers to be entered and didn't try to negotiate about a possible price, I guess there's not much to be done about it, unfortunately (but that really is nothing but my guess). Learn from that, though.
Selling the rights to your art completely, probably is less common and should definitely come at a higher price. Much higher.
From your position now, I would probably try to write him an e-mail in a kind tone that you thought about all this and you wish to basically clarify the rules and just agree on the terms on how your art is used and what the price is for this and that. Even if you don't have a contract it is always good to have the terms set and in an e-mail as a proof.
I know that is hard if the client was kind to you, but this just seems unfair. And honestly, asking if you wanted a souvenir from an overseas trip that you could potentially be on sounds a little like a slap in the face. His intentions may have been good, it is just how it appears to me as an outsider.
The fact that he collaborated with many illustrators honestly doesn't say much about his professionalism.
And even if he does give you a description or a reference to a character, or a photo collage, or tips on the poses, etc., the art is still yours and he shouldn't be free to use it however he wants, unless you agree. Which can be perfectly fine for non-commercial projects, but you should be aware of what the art is used for if he gains money from it.