It sounds to me like your kids are nearly grown. Even if he does decide he wants a relationship with them, they are old enough to choose whether to have one with him or not. I suspect that after all these years they are both angry and curious. Unless you have a rebel looking for "greener grass" letting them meet their dad (IF it happens) is probably not a horrible thing. (if you have a rebel, they may assume things will be easier with dad and you'll have a can of worms you don't want).
That support number is a heck of a lot higher. Most states impune interest. I had a $12K order in 1990 that was worth $55K 3 years ago (there were some additional arrearages too). 10% simple per year is typical.
You have a couple of choices, #1. Every state has an office that hunts down child support. It may be the county or the state but find your office and turn it over to them. It may take a while (they seem to like easy cases), but they can use his social security number and other public records to find him. They also do not charge for this, and most will seek an increase for cost of living (boy would that screw the ass).
#2 hire a private attorney, but then you will probably need a PI to find him. You might be able to find him for less than $50 on the Internet using one of the services available.
Beware,. there is a group called "Supportkids.com." They make is sound like they will do you this huge favor and hunt down the ex, but I swear, they must have friends high up in the states they practice in, because they border on a scam. First of all, they are nothing more than a collection agency and they keep 50% of whatever they get. So much for collecting money "for the kids."
Secondly, they cherry pick and put as little effort in as possible. A friend here with the state of Texas office that does hunt down child support told me that Support Kids goes to them to do all the work and that they do it for free (but I think they were beginning to charge such collection agencies a fee).
Third, they don't work to collect interest or ever seek an increase in support. They also won't go after spousal support or any other orders, like "half the uninsured medical." They will also turn it back to you if they can't find the parent easily. If they find the parent, they do make nasty phonecalls, that's about it.
Thus they do LESS than the government and keep half of what they collect for you.
I think you should go for it. Not only might it give you some breathing room, but it might provide some extra money for your sons' college. Don't feel guilty if you keep it for yourself either because you and your husband have been covering the kids all these years, any money you collect is just a reimbursement.
Take care.