@Sidge It IS ridiculous yes, and I thought that when both our DCs went to uni. (They are now in their 30s and employed, gainfully.)
On the one hand at 18 they are deemed adults, yet parental income dictates the amount of loan they receive. It is mad.
BUT that is the system and we all have to work within it.
I appreciate how tough it can be. At one point we were paying (topping up) both our kids' loans so they could pay for accommodation because there is s mall age gap between them. We went from being in the black monthly, to almost in the red .. It was equivalent to taking on another mortgage. I had to increase my hours of work ( was self employed) and we had to tighten our belts.
Having said that, I have the impression that the OP isn't that secure in her marriage and as her DH is in the forces, he may be an absent step parent too. But that's a personal issue. Other step dads would want to care for their step children, wholeheartedly.
@Iwasneveragoddess I don't understand your outgoings at all. If your DH is in the forces, why is he travelling long distances by car all week and why is that not classed as expenses?
My feeling is you could both tighten your belts- maybe buy cars that are more economical on fuel, cut back on the food expenditure or holidays for a few years - AND pull together as a family. You seem to want to behave as if you are not married to him.
And going back to my original suggestion- your son, if he's a typical 18 yr old - will be costing you a lot living at home. Our food bill shrank considerably once DS went to uni, as did the amount of laundry we had. You will save on those.