Jason, I read some of these answers and I disagree. You aren't nessecarily the heartless azzhole they describe you as, you have to take in consideration of your new family as well the old in all this and let's face it - the whole situation is kinda crappy. I had a big fight with an ex and she moved out with no notice, I tried to reach out to her for 3 years and got no response after messages, trying to find her on internet, through freinds, through her grandparents, I still don't know if she is even alive or had a son without my knowledge - I gave up eventually and now I have a wife and 2 kids.
With all that being said, the best thing to do is to keep trying to reach out to your son. You're new kids and family will see you differently if you just throw in the towel too early, you have to teach your new kids and the old ones that every kid deserves an awesome dad. My wife see's completely different than me of course, protecting her natural interests, but you have to look at it like this. If your new kids didn't have a father or you couldn't be there for them, wouldn't you still try to build an overdue relationship with them??? - OF COURSE MAN!!! You're new family may not give you the forgiving that the old son would, just keep working both sides and don't get discouraged - remember this whole situation is F'd up - set a good example for everyone how to handle something properly like this.