I'm just about 260 pounds and I've lost 30 in the past few weeks.
I've only tried talking to a few girls that I was interested in, but I've rarely tried to pursue anything further even if there were good "signs." I've always felt that my weight would end up getting in the way.
Yes, if he was determined to lose it and if I didn't see behavior that would indicate that he might lose interest in working hard, definitely. Weight is not everything. If he has a good heart and an awesome personality, I wouldn't hesitate to go out with a guy who's a little overweight.
Would you date a guy who was over weight, but trying to lose it? No as I doubt said guy who have even noticed me if I was overweight and likely he's pursuing me because he considered me attractive. So I see no reason for me to shortchange myself on attraction when he seemingly hasn't especially about a factor I doubt he would have overlooked.