Your brother needs to check what "Terms & Conditions of Agency" he signed with his Estate Agent.
At the very least, he can expect to pay for any marketing expenses incurred on his behalf.
Also there is a possibility that he will be liable to pay both his new Estate Agent and his old Estate Agent a percentage of the sales consideration acheived as many Estate Agents also insert a clause in their terms to this effect.
If he is dissatisfied with his Estate Agent, then he should express it directly to his agent and tell him that he is considering dispensing with his services. Perhaps this might give him the jolt he needs to improve his service.