I am surprised by most people saying attraction can't come back. I don't have experience of this, but it feels to me like attraction in a long-term relationship is normally made up of three elements:
- a 'newness'/ excitement of falling in love (no this probably won't come back, and that just eases with time for every relationship, though talking about it and trying new things can help)
- the other person's physical attractiveness (this can change if they change themselves)
- whether your emotional needs are being met by this person (while you say he's lovely person it doesn't sound like you are having good couple time and deeply sharing/ nurturing each other on this level).
So, I think counseling could help with these things. But it is definitely not an easy road. Also, Having struggled with my own weight for a long time, it is not easy to lose and change how you conceive of yourself physically. He won't enjoy being overweight - nobody dies. It is NOT just a case of 'he should make the effort'.
I was overweight and seemed to make very little effort in my last marriage for years. My husband had never struggled with his weight so interpreted this as not bothering. He got frustrated and made passive aggressive, derogatory comments every now and again, which cut me deeply. I felt very undesired by him and when I did make an effort or manage to lose a little weight, it seemed to make no difference - and that made me feel far worse and more rejected than if I had never tried. It was emotionally much easier to not try and be rejected than to try and be rejected. I used eating to fill loads of emotional gaps, and it became a sign of his lack of regard for me. Having our child also took a big toll. I was never huge but settled at an unhappy size 16, rather than the healthy size 12 I had been when I met him twenty years before. And while it seemed I made no effort it ate away at me and corroded my happiness every day. I was always trying something or other but became embarrassed to even tell him, because he would roll his eyes - he had heard it all before.
He never talked explicitly, with compassion about how it was affecting him - he just made mean, casual comments then insisted he was joking....and in the end he had an affair and I ended the marriage.
Ironically, since then, And with the help of therapy, I have lost all the weight and started loving my body, taking care of myself, enjoyed buying clothes etc.... honestly, I look pretty good now! I often think it must be very frustrating for my ex, who meanwhile has actually let himself go quite a bit.
I suppose I am saying all this because physical effort and weight are not simple things. They are symptoms of deep psychological stuff in yourself, and also of how your partner feels about you and treats you. There was definitely something in my marriage dynamic that contributed to my giving up on myself physically, which I was unburdened of when he left.... and perhaps the opposite for him. Counseling may be able to unravel this....
Last note (sorry this is so long) I would NOT feel guilty about talking with him explicitly (kindly) and trying to get him to change before leaving..... you owe it to him to give him a chance and worse case scenario, he gets healthier before you leave and he knows you both did your best. Just make sure you are also open to changing - it is you, explicitly, who is not happy in the relationship, but it is highly likely neither of you are and both of you need to change (in fact his weight and lack of effort may be his way of saying he is not happy either!)
Good luck!