So, based on general comments, I think you can give him some prompts that you want his attention while you two are talking before you start with whatever conversation you might have.
"Do you have the time to talk? You seem to be really busy."
If the conversation is going to be something particularly complex or something that he needs to remember, you can also ask if he has pen and paper or something to take notes.
I had a coworker that would call me to ask questions when he was bound to get interrupted during the call or in a hurry, like 5 minutes before a meeting, and didn't realize it, so I started prompting him to call his attention to the fact that he might get interrupted or it was not a good time for HIM to call. I did both the asking for the time and the pen and paper and it worked.
You can also reverse roles and say upfront that YOU don't really have the time and if he puts you on hold you break the call. When he calls you back (which he probably will), you'll just tell that since you didn't know how long the side conversation would take, you went your merry way and waited for him to call when he would be undisturbed. - This one is a bit passive-agressive, but it can work. It sends the message that your time is just as valuable as his is.
I tried this option on different co-worker that was immune to the prompting and was quite sensitive to candor, so I couldn't openly say "would you cut it out, if you call me and don't want to hear what I have to say, just don't call" or something along the lines, so I used the "passive-agressive" approach (not really my style) of dropping the call and waiting for them to call back and saying "yeah, I really didn't have the time to wait until you were available, so I just went to do my stuff and waited for you to call back when it was a better time for you". It worked too.