I'm really surprised by some of the very strong YABUs here and can only assume that most, if not all, of those responding in that vein have never actually been in a situation where you literally have no money.
The OP has said now more than once that they were totally strapped. Unfortunately, that means, no matter what your relationship to guests, and no matter how much you love them, that it can be extremely worrying and stressful to bear all the additional expense that comes with up to six extra guests at a time. You can't, just can't eke out your normal food shop for 2 adults and 1 child to cover off an extra 6 adults - and that means something has to give .... it often means going into debt for example. Which might be okay if it's short term and agreed beforehand, but not if you have no way of paying off that debt any time soon.
Furthermore, alcohol, for example is a nice to have .... it's not essential, and if you're literally broke, it must be even more worrying to see money being wasted on that (though in OP's position I simply wouldn't have bought alcohol at all).
It's all very well everyone saying "they're family", "he's their dad, he wants to treat them" and so on ..... but these are adults we're talking about and dad's desire to treat his grown up kids (who weren't short of money in the same way OP and her DH were) must surely have meant that his youngest child - who really was a child - would probably have missed out in some knock on effect way. Flip it round, surely a solvent adult child - who is aware of their dad's financial difficulties (OP said they were) would be very ungracious indeed and uncaring to take advantage of their dad without making any effort to minimise the effects of their visit. Many of us have probably experienced the "I'll pay", "no, I'll pay" dance that can go on between family members and yes, it can get awkward when that happens but good god, you don't have to use too much imagination to work your way round that if your dad refuses a direct financial contribution ...... gifts of wine, boxes of chocolates, definitely replace petrol you use (FFS), leave an envelope behind, take hosts out for meal, do a shop, make a meal one night ...... some, or indeed all of those things should be second nature in a loving family where there's a very clear financial imbalance and quite clearly, your stay would add extra strain. In fact, even if your hosts were rich, it's basic good manners to offer, even if it's declined. It sounds like none of these kidults ever offered a thing.
And yes .... it may well have been pride. I understand the dad feeling ashamed of his financial situation, and, I understand it's natural to want to spoil your nearest and dearest. But he had no business inflicting the resultant effects (which presumably would be OP and her son going without in some fashion and/or debt) on his partner. When you're with someone you can't make unilateral calls like that.
I'd have felt the same as the OP in her shoes. I think that regardless of the dad's desire to play mine host I'd have also felt very angry at the adult kids who were happy to take full advantage of that without so much as a murmur of appreciation and/or concern for the impact their visit would have. I repeat ..... this does not, or should not, have any bearing on how the parties concerned feel about each other. The fact remains, you can't get blood out of a stone - and that's a reality for many families, who, if they were better off, would otherwise love to throw their doors open wide and put on a fabulous stay for guests, rejecting all offers of help. FWIW, however well off I believe a host to be, I'd always bring some sort of gift, at a minimum simply to say thanks for being invited at all - and would certainly offer to pay my way for longer stays. Where I was well aware that a host was struggling financially I'd do everything I could to ensure me being there wouldn't add to their problems - surely all you YABUs must see that's the decent thing to do ?