I'm going to have to say this is more of a mental situation.
I have come across this situation many a time and I have found that being in the correct mental state is all you need.
I could never ejaculate during oral sex for some reason, and found that I had a mental block on how it would feel to/ or if it is ok to "bust" during this time.
By really getting a mental visual on how it would look and feel to ejaculate in the situation is how I got past this.
If you know 100% that you want to bust and have that feeling, picture what that feeling is like, and just enjoy yourself, you will have no problem ejaculating.
A lot of people focus on pleasuring their partner too much and forget how to get the correct pleasure themselves. Learn your body and learn what it needs to "go". Trust me, if anyone is turned on enough they will be able to ejaculate, just learn how to get to that point.
Stop thinking so much and just do what you enjoy.
I hope this helps!