Did you discover the gender?
"would the ultrasound tech ... know what he was doing?" - I imagine so, but I think the estimated date of 5th Feb was out. I think that maybe 40 weeks were added to the estimated conception date instead of 38.
I am not saying your ex definitely is the father, but that nearly all the information you have given me leads me to that conclusion. The only bit of information that is out-of-line is the due date of 5th February. The fact that you believe in your heart your ex is the father doesn't add to the evidence!
Your doctor is saying that the next ultrasound can be compared with an earlier ultrasound to give an indication of how much development has taken place between the two scan dates, as well as the total amount of development as at the last scan, thus giving a more accurate assessment of the due date (and therefore the date of fertilisation).
The due date of 5th February is a projection made by the technician, and that projection is not usually changed until there is evidence to do so.
Whether you baby actually arrives on 23rd January or 5th February is largely beside the point. You are anxious to determine the most likely date of conception. You want to work backwards rather than forwards. Conception date can only be estimated by using certain data, and for that to be robust, consistent treatment of such data is important. It is because of the difficulty of ascertaining the timing of the different (Hormone-driven) stages of the menstrual cycle, particularly ovulation date, the LMP start date is usually used, and allows a typical (but by no means universal) 2-week menstrual/follicular (pre-ovulation) phase. So 40 weeks is calculated from LMP. However, this phase is the most variable. The luteal phase (ovulation to next menstrual period start) is generally the more consistent, being usually about 14 days.