@Bonsaitreepic
Your son who is now an adult needs to get a job, or give you the bulk of his Jobseekers Allowance (maybe £200 a month,) to try and make up the shortfall. Or you need to work more hours yourself. (Preferably both!)
There's only so long you can keep claiming benefits for children, and depending on them. I don't mean to sound cruel or harsh, but I have heard this story SO many times from people who largely depend on UC/housing benefit/child allowances, and then when the children are no longer children, it stops/is reduced a lot. Then they are shocked and stunned that the income is now less, and expect some magic fix by posting about it on the internet.
Your son is an adult now. He needs to get some income in. OR as I said, you need to earn more. It's an easy trap to fall into, being dependant on benefits, and I am NOT attacking you for it, and you shouldn't feel bad for it (as you said you sometimes do.) But I can't see the point - when the children become adults, and the benefits are pulled/cut - in people saying 'help! I am now £300 a month down.' I am not sure what they expect people on here to do. The answer is obvious. The child - now adult - needs to step up and hand some income over!
I wish you luck and don't mean to be rude, but yeah, this is something that is not going to resolve itself. They are not going to just magically re-start paying £300 a month more. You - and your son - need to bring more income in between you, to make up the shortfall.