I'm wondering what sort of a relationship he had with his mother. Sometimes, if a parent has not been very loving, children (of any age) will have disproportionate feelings of guilt and responsibility that more secure children do not have.
It could also be, as others have said, that he is not happy in France and his mother being ill gives him more justification to return to the UK.
Or maybe he reasons that she is very old and is unlikely to live much longer, whereas, hopefully, you and he have several more years left together to enjoy yourselves when she is gone.
Whatever the reason, I do think two weeks a month is excessive and, much as he is to be admired for not just leaving it to family in the UK to take responsibility, it does seem unfair to me that this has been at the expense of your happiness and peace of mind.
You need to hve a proper talk with him and explain how you feel. You need to find out exactly where you stand and if it truly is concern for his sick mother that keeps pulling him back to the UK. If you are satisfied with what he says, perhaps you can reach some sort of compromise - e.g. he only spends one week a month there or you go back with him sometimes (if, of course, that would be satisfactory to you).