Mega Fat Loss Secrets

I have a 37 pound Australian Shepherd. He’s a little over a year old. We take him on hiking trips and until now we’ve been carrying his supplies; water, food, etc. I’ve ordered him a hiking pack and was wondering what would be a good weight, at max, he could carry. I want to start him gradually and not start with the full weight, but in the end what is a healthy and SAFE weight for him to carry.