I'm really sorry.
It sounds as if your fiancé was taken by the cleaners before you or his mates were. Consequently, he holds his finances to himself, selfishly but there is a fine line there in my opinion. To not answer or more so, to not "offer" to pay for petrol so you can see your Son. To not help with food. To not take in your son as part of the family and have him do odd jobs around the house until he obtained a job.. It's all very selfish.
I'm probably the same age as you if we are going to go off of "64".
My ex husband was simular. He held all his money to himself even when we travelled, he would take out 1/3rd of the expenses of dinners and sight seeing only and didn't even pay for any of the trips. He had been married before and lost out in Divorce, mind you, his ex wife went from size 8 - 14 within 1 year, I often wonder if that was comfort having lived with him for 7
He told me he demanded that she go to the gym or he'd Divorce her, well she up and left and Divorced him
He paid the Mortgage, I assume your fiancé is paying the rent/mortgage and that's it (so that he would have it on paper that he was paying for "his" house, the amount per week I paid for everything else was higher . I ended up realising that no matter what I said, what I did, even bought him a motor-bike, nothing could change his mind that I had no intention of taking his money.
This man wants someone in his life. He wants love probably doesn't show much does he, doesn't know how to, may even secretly dislike women, they are only there for a few things. Damned if he is going to pay your way or your sons, it's his money.
I'd like you to consider who you are engaged to truly consider the love shown to you, the togetherness. Your son I'm sorry is blood, he's worth more, he will stay with you for life.
Wishing you the best for Monday and hoping you will have a reality check. He is your fiancé, a bit of gas isn't going to kill anyone, he must know that your son is important to you. If he doesn't offer, is this really the guy you want to spend the rest of your life with? No unity, togetherness, equality.