How regular were your cycles up until you got pregnant? If every single menstrual month in your life has been 28 days long, year in year out, and your menstrual months never vary by even one day, then if your period came on the 1st, you would have been past ovulation on the 19th, by 4 days. However, if your menstrual months vary (many women's do) or are not 28 days long but regularly some other length, you could have ovulated earlier or later, including on the 19th.
When did Mr. 19th tell you he thought you were pregnant? If it was once you told him that you are pregnant, his comment is not as significant as if he mentioned it at the time. It's easy to re-cast something as important in retrospect when faced with a possible consequence, and to believe it, too. Did he say on the 19th, "Gee, you seem different, are you pregnant?" or did he just say it after you told him you were pregnant? ("Well, how do you know you weren't pregnant already, I think, uh in fact, I'm sure I remember you seemed different.") Not saying he's lying, but memories are malleable in retrospect when something dramatic has happened. People look back and try to make sense of little cues, or just grasp at straws.
If you were already pregnant on the 19th, it would have been by just a few days, (unless of course you ovulated very early, but you didn't mention having sex with anyone earlier than the 11th). If you got pregnant that day, you would have only been 8 days from conception when you had sex on the 19th. Nobody has ever written in (to this community anyway) that their body seemed different to them or to their boyfriend, as early as that. It would be only two days after implantation, with so little hCG in your system that if you had taken a home pregnancy test it would not have given a positive result. (I had a blood test on day 8 when I was pregnant with twins, and it was negative.) That's not enough hormones to cause body changes. In fact the most common remark women make about their earliest few weeks of pregnancy is that they didn't feel any different at all.
My guess is, any body changes Mr. 19th detected might have been from something else. Water retention, weight gain, dietary changes, sleep-pattern changes and other hormonal things (including steroids from the ejaculations of Mr. Partner on the 11th, 13th, 14th and 16th) can affect the body and might change how it looks or feels. If Mr. 19th thought when looking back that he had sensed a difference, it could just as easily be from one of those causes (if he really remembers a difference at all). It could even have been due to your vaginal areas being swollen from all the recent workouts with Mr. Partner.
We've never had anyone write into this community that their guy could tell they were pregnant by the way their body felt. Not saying it doesn't happen, it might be that a couple of months into a pregnancy some guys notice a "glow" or some tummy. But my guess is that it doesn't happen as immediately as it would have needed to happen for him to correctly assess this fact on the 19th.
So I assume that means the question of whether he has the chance to be the dad is still open? Start by assessing the regularity of your menstrual months. If you're lucky and they are always always always 28 days long, you can probably rule him out. Get a DNA test when the baby is born and then forget it. If your cycles are irregular, you will definitely need the DNA test. Yes, your boyfriend has the odds here, not just the amount of activity but its timing are all suggestive he is the dad. But no, the other guy is not entirely a thousand percent ruled out by the information you have explained so far.