I had two echos when I was pregnant. One at 23 weeks because I had a SUA, we didn't know about the DS. That one was perfect. Then at 35 weeks because she had stopped growing and they were going to induce me. That one showed a perfect heart.
After we found out she had DS at two weeks of age, my ped suggested we go to a pedicatric cardiologist just to check.
Well, it turned out she had a HUGE VSD. They missed that and the DS prenatally.
But since your guy has gone 2 years, I imagine he is fine. Seems they would have found something by now. Praying it all goes as expected.