Based on the information given, I highly recommend going to a vet because your rabbit’s immune system will not be able to fight off fur loss that’s accompanied by yellow crustiness, dryness (which can lead to breaking skin by the mouh) and potentially an actual bug infestation. My Wick is very susceptible to ringworm, and the fur loss and condition will only get worse for skin-related conditions if left untreated.
It could be that an underlying condition which is causing the weight loss lowered your rabbit’s immune system, making him susceptible to a skin infection. This can be a possibility for weight loss. You mention he hasn’t gained weight, but if he’s fully growned, maintaining the same weight is healthy, as long as it’s a healthy weight. If he’s underweight, then that’s more of a consideration.
I’m glad you’re seeing a vet asap. Sending you good vibes!
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.