Hey hon :) Usually an aversion to anal is tied up in misinformation; that anal is painful and dirty…or makes you gay, etc. It may be that he is okay with sucking his buddy off but getting fucked in the ass may be too much for him to handle. Perhaps encourage him to talk to you about why he doesn’t want to give it a try. You may want to show him the link below…everything a guy needs to know about getting fucked in the ass. It should help to put him at ease. Good luck!
Anal Sex Tips For Men