Well he's as bad at maths as he is food shopping because £200 pm isn't even £50 a week, it's £46 pw averaged over the year.
Like everyone else has said, he's asking for the impossible - an interesting varied diet on a very basic budget plus 'snacks'. You could probably feed the pair of you on a rice, beans and eggs type diet, but unless you absolutely can't afford any more, you shouldn't have to do this.
Plus if you're being fair to both of you, you should end up with the same amount of personal spending money, not split bills 50/50 when he earns more, unless you're deliberately choosing to earn less without otherwise contributing to the household, eg by doing more cleaning, cooking, shopping, budgeting, laundry etc - out of interest how is this split?
But the fact that he refuses to be reasonable over grocery money and there's constant arguments suggest that he's possibly financially abusive - indirectly restricting your access to money by refusing to pay his fair share.
You could try telling him to be responsible for the grocery budget, but it needs to be for more than a few weeks, otherwise he'll spend less than the budget and you might not notice until you start running out of things like washing powder, toiletries, tea bags etc.