Looks like some people need to rinse the bitter out of their mouth. There is some major conclusion jumping and speculation that is frankly no one's business on here.
OP, if you are still around why not see what kind of deal could be made? You did say that your ex does see your son and is good with him. I know that just seeing him isn't enough. He isn't following through with total parenting which is also financial contribution which especially includes health insurance and I'd be pissed too. Is there some compromise that could be found? Force his hand on the health insurance and then freeze what he owes or something?
I personally wouldn't listen to my mother and I wouldn't listen to my ex. I would talk things over with my current husband, who is taking the responsibility that the other father should be doing, and I would think about the relationship with my son and his father. Some things are more important than money no doubt but there is also a responsibility to be a total parent and not just when it's good for someone. Good luck to you.
I totally agree!
I am not going to judge the OP or any of the other DISers who've posted a reply here, but I will say that I am in a sort-of-similar position as the OP, but with some differences since my DSs are older now.
My DSs are now adults (ages 21 and 18) however both of them still rely on DH and I, fully. (My DH is not their father, by the way).
DS21 has had some very serious mental health issues (I've posted about this within the last few months) and lives with us; we provide everything for him while he's on the road to better health. (We pay within reason; all of his basic neccesities are covered here, plus we cover his doctor co-pays and medications, and give him a few $ in gas money for doctor appts and job-hunting). My first EX payed child support faithfully but that ended when DS dropped out of college. Legally, that was his right, however my DH and I continue to support him for the reasons stated, and we realize that this is OUR choice and while it'd be nice if my EX paid a little towards DS21's expenses, we realize that he is not obligated to do so. DS21 may start taking a college course or two in January, and if he does, we will pay for the class and his books. One step/class at a time with him.
Now on to DS18. He just graduated high school in June and has been going to university full time (and doing pretty well.

). He works around 20 hours per weekend at a restaurant near his school. My EX #2 has become physically disabled in the last couple of years and he and his wife (they have no other kids, she had none previously, and they had none together) manage to get by on his disability which is substantially less than what he made while working, and her job in a nursing home. They contribute a small amount to DSs living expenses, but DH and I are paying fully for his college education (tuition, books, gas since he's a commuter). EX#2 does send a few $$ here and there, when he can, to help DS18 pay for his car insurance, or whatever. I think that's awesome, and so does my DH.
My point in all this is to say that I'm EXTREMELY GRATEFUL to have a DH who is NOT the father of my DSs who has been willing to pay for things for us (I was a SAHM for 18 years, 9 1/2 of which were while married to DH) even though is not legally obligated to in any way, shape, or form. If it weren't for my DH, then my DS21 would be on the street or in a hospital setting, and DS18 would be racking up students loans. (DH and I believe in avoiding student loans if at all possible!).
Now having said all of THAT, I want to say that there is no way in hell I'd ever have either of my EXes put in jail for nonpayment of child support, in MY situation. I've felt that my DSs relationship with their father was AS important as their relationship with me. I honestly don't see how going to jail would ever HELP a non-paying parent; and certainly would hurt the parent/child relationship. Unless the thought of jail would be the motivating factor to make the parent find SOME way to come up with child support. But the OP said her EX wasn't a bad guy...so I don't know if that's the reason, in this case.
IF I would've been a struggling single mom on my own and was receiving no child support, I'd be upset if I felt that my EX wasn't doing enough to provide it, and I've have to persue things further simply to exsist. However, from what I've read here, both myself AND the OP have a DH who is able to, and does/did, provide for us and our children from previous marriages. In this case, why NOT be lenient, for the sake of the relationship between child and father.

The OP's situation is what it is; her EX should've been looking for more stable work for the past year, but the fact is, he hasn't. You have to deal with the present.