Check with the rescue to find out what he was eating before he came to you if it was something different. The hay is great (especially fresh hay) but he will need more than that. Variety is great.
I'd also try some fresh grass (pulled and not cut, and from areas not used by dogs or wildlife) and I'd keep trying various bits of veggies.
I'd definitely avoid the Barbi Brown food. They have different goals for their rabbits. You really don't need to be conditioning flesh or putting on weight.