He would be crazy to accept that fight. Even if he could lose the 45 lbs in time, he would have zero energy in the fight. If you're an up and coming fighter it MIGHT be worth it for a big shot but if you're an established name, you have nothing to gain from doing that.
As far as staying in fight shape year-round, that's easier said than done. We also know he's trying to be an actor more than a fighter lately. I don't blame him. More money and less pain.