So sorry to hear of so many AC who are full of gripes about their parents! My DD has had her issues w/ me but not to this extent. I guess this is another reason I should consider myself lucky. Hugs to all of you dealing w/ this problem!
A lot of these issues seem like cases of "mismatched expectations." IMO, a lot of these AC have a different idea of what mums/parents are supposed to do/not do than their mums/parents have. In some cases, they may need to be told that you (general parent) expect them to respect your life choices (career, degrees, etc.) the same way they expect you to respect theirs.
Mothercat, I agree w/ PPs (previous posters) that you should stop trying to "win" w/ this DS (dear son). Just be who you are and let the chips fall where they may. Well, except that I agree w/ your decision not to offer advice about the baby, etc. DS and DIL are adults, and now parents. Let them do things their way, even if you disagree. You can have your own opinions in your mind, of course, but unsolicited advice will likely be seen as "interfering." You can show "interest" in what they're doing ("How does that work?"... "How are you decorating the nursery?" etc.) w/o criticizing or offering advice.
As for not saying yes to certain financial requests, I don't blame you. Did you explain to DS at the time that you simply couldn't afford it? If not, if the topic comes up again, can you point that out now? No need for elaborate explanations, just "We couldn't afford it" or the like would be enough, IMO. I think that about the other AC mentioned here who seem to be dissatisfied w/ what their parents provided, etc.
But yes, perhaps the blame fell on you b/c DH stood in the background, letting you relay the decision. Maybe DS thought these were your choices alone? Perhaps DH should step up now, at some point, and make sure DS knows these were joint decisions, if the topic comes up, that is.
I must admit, I'm a little confused. You and DH seem to think DS' attitude is related to lack of financial efforts, but DS says he has had to "reign (you) in." That sounds as if you said or did things he thought were out of line or whatever. (NOT saying you did, just that he might think so). What kind of things did he believe he had to "reign in?"
IMO, sometimes, DS may just be expressing his tensions, etc. That would explain, for example, why he recently sounded off to you about the family not helping - and then turned around and thanked you for your help. He was probably feeling pressured, etc., the first time, and might have been talking about other family members (maybe even DIL's family), not you. The second time, clearly, he was calmer and focusing solely on what you had so kindly done for them.
Excellent suggestions from Smileless, IMO!
And, of course, congratulations to all and many joys!