What an awkward situation - and 8 stone is tiny.
For a number of years, and certainly since I have been in a relationship I have been awkwardly underweight, partially intentionally, partially luck of the draw. There was a brief 6 -8 months where I ate loads more, exercised a lot, and lifted weights and my body really did change and I looked and felt better - and it was clear that the byf found me more attractive.
So now I am tall and awkwardly underweight and it is not very attractive, especially facially, and makes me feel awful. But until I feel able to deal with the stress of the final phd and no job situation, eat loads more, and start doing exercise/weights again, I can't change that.It is very annoying when I think that his body his well-proportioned.
As the less attractive too-skinny person in the couple, I am just grateful that he is patient and doesn't say anything negative (beyond being too thin, he has never said I am unattractive) but i can tell that he thinks I am less attractive now, and I feel that way too. So first on the agenda after the thesis is weight gain.
So I think, generally, that i most couples there are the body politics of "who weigh's what, in relation to whom.." and how changes are managed over time. My parents, for example, got quite rounded in their middle age, but BOTH decided to start going for walks, eating better, and going swimming. They are now the clothes sizes they were 20 years ago, before their 4 children came along and ruined that ;) So I think it is important to be sensitive and not do what your boyfriend did and target your weight, though he may well be conscious of his. Weight is surely the minefield of a relationship at points - especially post-christmas. I think that if it is at all possible, it is nice in a relationship to get to the point where one never feels judged about their appearance and feels relaxed. and importantly, never makes their partner feel uncomfortable in theirs.