I don't know that anyone is willing to take their fish out if the bowl to weigh it, to tell if he's fat/starved when you look at your betta from above he should be a long teardrop "the shape of a carrot"
I suppose you could weigh a jar with and without the fish and find the difference, so it wouldn't stress him~ But the reason I wanna know is I'm calculating filtration capacity and need to know the average weight of bettas in general to do that. Might be a bit overkill to calculate it but ah well xD
People dont bother weighing little fish, its just not worth the hassle... But if you really want to know Id put him in a bag/cup and onto some scales for a couple of seconds. Then after youve put him back in the tank weigh the cup and deduct the cup weight from the total weight.
Fish wont be happy with you but he'll get over it, they can handle being out of the water for a few seconds