I've been told that you cannot visualize the cord sufficiently on ultrasound to see what's going on with it -- you can see little sections, but not the whole thing, and you usually can't tell if it's around the neck, etc. Particularly late in pregnancy, when there's so little room in there, it's next to impossible to see much of the cord, since the only part is visible is the part floating in open pockets of fluid... as I'm sure you know, there are very few open pockets of fluid at the end.
If they were doing biophysical profiles and or non-stress tests during your pregnancy and they were all normal, they would have had no reason to expect any problems. Since it's difficult or impossible to really assess the cord late in pregnancy and since there's no test to see how well the placenta's functioning, they do the BPP's and NSTs to assess for fetal well-being and hypoxia. If the baby shows no signs of hypoxia (lack of oxygen), they assume the cord and placenta are working well.
Now, if there were signs of hypoxia and distress in your BPPs/NSTs and they didn't do anything about it, that would be malpractice! I don't know if that's the case, but it would be good to gather your records and ask someone to go through them (your OB would have the skill to, if you trust him/her).
You can definitely speak to your OBGYN about it or even a lawyer if you'd like, but in my understanding (which may not be perfect! This just is what I've been told and gathered from the OBs and maternal-fetal medicine docs I work with) of what ultrasound can do and how it's used, I wouldn't think it was malpractice based on what you said alone. But of course, I don't have your medical records and don't know the whole story, either. I wish the best for you and your little boy!