Well done on correcting things so well with the hay-only diet! It takes some tough love, and that’s not easy with these beggars.
It is important the weight loss doesn’t happen too quickly. You could add in some other types of hay to see if that helps maintain good weight. Mix in some oat or wheat hay with his timothy perhaps. For other nutrition, maybe occasionally give oxbow supplements? I think sherwood may have a product also. Keep in mind, rabbits don’t need a ton of nutrition. They were designed to be able to process lots and lots of low nutrition food, and make the rest themselves.
You may actually be able to bring in more of the food he had before (limited) after his gut has had time to re-set and become more balanced. That can take a few months. You could even give a probiotic like benebac to help things along.
His droppings look completely normal and his behaviour has actually changed for the positive with the diet switch (with him being more active throughout the day)
That’s awesome.