In England & Wales he can change when on legal aid if there is a good reason, but it isn't something to be done lightly. Your boyfriend would need to find another solicitor to take on the case before leaving his existing solicitor and it can be difficult to find a new solicitor because there aren't enough of them willing to take on cases from other solicitors.
The new solicitor would then require to get up to speed which effectively means there will be some duplication in work and costs.
The Scottish Legal Aid Board advises that a change would only be considered reasonable in exceptional circumstances such as a solicitor dying or retiring.
Your boyfriend is entitled to good advice and can complain to a designated person within the legal firm. However legal aid solicitors can sometimes only spend a limited amount of time on a case and it doesn't mean the solicitor isn't any good. Also just because a solicitor doesn't tell him what he wants to hear that doesn't necessarily mean they are bad at their job.