Hello! Apologies for the delay in my reply.
In my experience, enthusiasm is nice in a healthy amount and you can show that in your character’s expressions. If someone is into another person like that, they’re gonna look interested! That’s normal and healthy! Checking if the other person is sure is also another good way to avoid those stereotypes. Confirming consent is always welcome, both IRL and in your writing.
So, show the guy being into it if he’s down, but balance that with some uncertainty or a consent double check to tone down the “guys always wanna bone beyond all else” crap that runs rampant in the media–even for something casual.
He can even say no without slut shaming if that’s the route you wanna go. There are many reasons to not have sex beyond “it’s a sin” and you can just have him explain his reasoning. “I would like to get to know you better first.” “I don’t feel well right now.” “I appreciate the offer and I’m definitely into you, but I’ve got a lot on my mind right now.” or any other reasoning your character may have for not wanting to engage.
Hope this was helpful!