a vagina and ass are two very different things - a vagina will respond to being turned on, an ass needs to be warmed up (depending on the guy) a little to a lot, so foreplay is required to loosen the guy up. don't believe porn where a virgin takes dick with just a bit of spit and it goes straight in!!
its easier for a bottom guy to relax if he's in control, hence easier if he sits on you and rides you, as he's in control and can manage how fast he takes your dick and how deep you go in and he can jump off real quick if too much too quickly. Bottoming takes a degree of trust and if there's no trust then he'll be tighter than.....
so you need to learn how to loosen his ass, play with it, or get him to play first (dildos or whatever he likes) to loosen up, a bit of lubed up fingering of his hole and gently pushing in as he relaxes.
but unless he's very experienced and you're not huge, the majority of bottoms cannot just take dick up there ass with no prep first.
if you're the top you need to ensure he's relaxed, ready, trusts you etc Being a top is hard work;-)
good luck bud