It happens. My ex only went to 1 ultrasound. The only reason he went was to find out the sex of the baby. Then he got pissed when DD wasn't a boy. Which is sad cuz we were engaged at the time. Anyways, I hope the baby is healthy! Congrats on the little boy!
You didn't go alone, you had support!!! Odds are he probably felt uncomfortable with a 3rd person there or it just isn't important to him. It's hard to accept that everyone is an adult and you cant control their actions. (trying not to be a Debbie Downer here) The only thing you can force out of someone is what they are legally obligated to do, and even then it's up to the court system to enforce it. It sounds to me like you need to start looking at who you have as a support system and move away from beating yourself up. It might be time to accept the fact that you not 'we' are having a child.
My X never went to anything... no appointment, ultrasound, or even the birth of our daughter! I'm so sorry! But you have to focus on being happy and feeling the love from all those who are there to support you!!
I'm not a single parent, but I was a single pregnant woman (I placed my child for adoption when my ex left me, he was seeing another woman, whom he also got pregnant).. anywho.. I went to most of my Dr.'s appointment alone or with the adoptive parents. It sucked. Hang in there.
BM to Kenzie 9/1/04 --- Married 1/22/09 ---
Me 27 - DH 25 --- TTC our first since April 2010
Didn't show up. 
Re: Just went to my u/s alone
I'm so sorry! But you have to focus on being happy and feeling the love from all those who are there to support you!!