Lots of questions here (that you obviously don't have to answer publicly) but how long have you been together? Are you renting or do you own? Who's name is on the mortgage? And I assume by "DP" that you are not married.
My first live-in partner earned 2x my salary so he willingly paid 2/3 rent then all food/bill/household outgoings were split 50/50, all paid into a joint account. He paid everything to do with his car, and I eventually paid a supplement on his insurance (and fuel) when he added me. We saved 50/50 for holidays and weekends, etc..
I am having an ongoing discussion with my current partner as we earn roughly the same but are in different financial positions and he has a child. We are trying to settle on a level of proportionality that reflects both incomes and tries to mitigate the costs of 50/50 custody on my finances (that is not meant to sound heartless: nothing to do with the child but a regonition that I'd have to live in a more expensive area with one "extra" bedroom that I'd not specifically need, plus teenagers can bankrupt a food budget in under 12 hours).
In your situation I think you need hard data. It sounds as though the current split is unfair and increasingly so.
You need figures for all the outgoings, and you need to know precisely how much of these costs you are paying:
-rent/mortgage
-council tax
-electric/gas/heating
-cars (even if not shared ownership, they are used communally)
-food costs
-takeaways
-savings (bet this is split too)
-holidays/weekends away
I understand a proportional split in rent if there is a significant difference in earnings, and if there is a mortgage you need to be very careful about who's named and who's paying. However, I don't understand why an adult sharing a living space would not pay 50/50 on household and food bills. Same with any shared/communally used cars. Same with holidays (unless the higher earner has luxurious tastes and should probably sub their partner).
Once you have these figures, you can sit down with your DP and say that you don't feel the financial split is fair. You can ask him what he thinks would be fair and how you might be able to agree this between you. If you can reach an agreement, super. However, you may find that he tries to put you off the scent and in that case, fall back on your figures.
My fear is that hiw cash spending is untraceable, so he could well be saving up lots of money while you're paying for his day-to-day. He might also use his "I prefer cash" to say you're trying to force him into a financial agreement, but preferring cash in day to day use does NOT preclude him contributing to a joint account for household bills et al, or paying you back if you've paid his way 3 times in a row!