Being not so tactful seems to be a popular problem.
I know I'm guilty of it often as well, though I usually realize it after a while.
I think you might find it hard to convince most people to fill out post date surveys though.
If his weight is that much of a problem for you I don't see why you should even bother with the date.
You already have 2 traits that you dislike about him, that are enough to question your date.
Are you going to give him a fair shake after that?
That said, give him a chance.
Depending on his height and amount of muscle 300 isn't exactly huge for a man.
I know because I'm almost there myself.
However its not something that he couldn't fix in a year if you two got serious and it was still a problem for you.
Speaking personally, I know I'd take much better care of myself if I had a girlfriend, so you may not even have to do much encouraging for him to lose the weight.
In the end its up to you though.