Hello
Here is an important thing , if your partner is not an
HIV infected , NO chance of getting infection by you.
If he is HIV infected , there is chance of getting HIV infection by you is much more.
Ejaculation matter but this is not only a way of transmission of HIV infection because HIV has been demonstrated in SEMINAL FLUID both within infected mononuclear cells and in the cell free state.
There is a strong association of transmission of HIV infection with RECEPTIVE ANAL INTERCOURSE ( as you are ) , probably because only a thin, fragile rectal
mucous membrane separated the deposited semen from potentially susceptible cells in and beneath the mucosa and trauma may be associated with anal intercourse ( however , this is in detail but it was necessary so that you can understand in better way ).
So , a big IF is there , if he is HIV infected there is chance of getting infection by you is much more .
And about ejaculation I mentioned .
So get in P 24 test test nucleic acid amplification ( rapid )after 2 weeks of initial infection.
Good luck.