You are doing just fine. Really. Stop stressing out so much! Regarding the box for weighing.... IMO if it is stressing him out, it's creating a negative experience for him and since you're the one making him go in it, he could connect the scary box to you which doesn't help with trust. It is important to keep an eye on their weight but it sounds like he is gaining so you could probably wait awhile before weighing again as long as he's eating. Is there a perch on the scale or is it just the flat thing? My boys will not get on the flat surface of a scale so I bought one with a perch attached and they go right on. Maybe something to consider. You can also buy small portable perches that would just sit on the scale you already have. It would be less scary than a box.
On the eating - given the opportunity to eat, he is not going to starve himself! You're doing a great job offering different foods. I would definitely offer something consistent as well as a few things to change it up. If he likes chop, give him a spoonful of chop every day so you know he'll eat that even if he ignores new things. Make sense? Also, try offering the new things 1st meal of the day when he's likely to be the most hungry. And do try showing him yourself that these things are good. They do learn that way! I would also maybe sithhold the seed mixes until a nighttime snack after he's had the opportunity to eat the fresh foods. Seeds are not bad but kind of considered addictive and I know my boys would eat a bowl full of seeds before they'd touch anything else if I let them!! Since they're so favored, I keep them as treats/bribes/etc. and not a part of their daily meals.
Overall, keep being patient. Give it time. It can be frustrating but he's learning so many new things right now! Most of all he should be learning that you are trusted and safe and everything else will fall into place.